Using PDFium VCL in Delphi for side-by-side comparison of PDF documents

Comparing multiple PDF documents side by side is essential for revision review, quality control, and document verification. The Split View demo shows how to display two or three PDF documents simultaneously using PDFium VCL.

Overview

This advanced demo displays multiple PDF documents in synchronized views, allowing users to compare documents visually. It supports both two-way and three-way comparison modes, with independent navigation and zoom controls for each view.

Key Features

Two-Panel Mode – Compare two documents side by side
Three-Panel Mode – Compare three documents simultaneously
Independent Navigation – Navigate each document independently
Synchronized Zoom – Apply zoom to all views or individually
Text Selection – Select and copy text from any panel
Save as Image – Export current view as an image
Active Panel Highlighting – Visual indication of the active panel

PDFium DLL Requirements

Before running any PDFium VCL application, ensure the PDFium DLL files are installed:

pdfium32.dll / pdfium64.dll – Standard versions (~5-6 MB)
pdfium32v8.dll / pdfium64v8.dll – With V8 JavaScript engine (~23-27 MB)

Installation: Run PDFiumVCL\DLLs\CopyDlls.bat as Administrator to automatically copy the DLLs to Windows system directories.

Setting Up Multiple Views

type
  TFormMain = class(TForm)
    // Three PDF components
    Pdf1: TPdf;
    Pdf2: TPdf;
    Pdf3: TPdf;
    
    // Three view components
    PdfView1: TPdfView;
    PdfView2: TPdfView;
    PdfView3: TPdfView;
    
    // Container panels
    ScrollBox1: TScrollBox;
    ScrollBox2: TScrollBox;
    ScrollBox3: TScrollBox;
    
    // Splitters for resizing
    Splitter1: TSplitter;
    Splitter2: TSplitter;
    
  private
    FActivePdfView: TPdfView;  // Currently active view
    FAllViewsMode: Boolean;    // Apply actions to all views
    ThreeViewMode: Boolean;    // Three-panel mode enabled
  end;

type

TFormMain = class(TForm)

// Three PDF components

Pdf1: TPdf;

Pdf2: TPdf;

Pdf3: TPdf;

// Three view components

PdfView1: TPdfView;

PdfView2: TPdfView;

PdfView3: TPdfView;

// Container panels

ScrollBox1: TScrollBox;

ScrollBox2: TScrollBox;

ScrollBox3: TScrollBox;

// Splitters for resizing

Splitter1: TSplitter;

Splitter2: TSplitter;

private

FActivePdfView: TPdfView; // Currently active view

FAllViewsMode: Boolean; // Apply actions to all views

ThreeViewMode: Boolean; // Three-panel mode enabled

end;

Switching Between View Modes

procedure TFormMain.SpeedButtonThreeViewClick(Sender: TObject);
begin
  ThreeViewMode := not ThreeViewMode;
  SpeedButtonThreeView.Down := ThreeViewMode;
  UpdateLayout;
end;

procedure TFormMain.UpdateLayout;
var
  TotalWidth: Integer;
begin
  if ThreeViewMode then
  begin
    // Three view mode: equal thirds
    ScrollBox1.Align := alNone;
    ScrollBox2.Align := alNone;
    ScrollBox3.Align := alNone;
    
    Splitter1.Visible := False;
    Splitter2.Visible := False;
    
    TotalWidth := ClientWidth;
    
    ScrollBox1.Left := 0;
    ScrollBox1.Width := TotalWidth div 3;
    ScrollBox1.Height := ClientHeight - PanelButtons.Height;
    
    ScrollBox2.Left := ScrollBox1.Width;
    ScrollBox2.Width := TotalWidth div 3;
    ScrollBox2.Height := ClientHeight - PanelButtons.Height;
    
    ScrollBox3.Left := ScrollBox2.Left + ScrollBox2.Width;
    ScrollBox3.Width := TotalWidth - ScrollBox3.Left;
    ScrollBox3.Height := ClientHeight - PanelButtons.Height;
    ScrollBox3.Visible := True;
    
    SpeedButtonOpenPdf3.Visible := True;
  end
  else
  begin
    // Two view mode: equal halves
    ScrollBox3.Visible := False;
    Splitter2.Visible := False;
    
    TotalWidth := ClientWidth;
    
    ScrollBox1.Left := 0;
    ScrollBox1.Width := TotalWidth div 2;
    ScrollBox1.Height := ClientHeight - PanelButtons.Height;
    
    ScrollBox2.Left := ScrollBox1.Width;
    ScrollBox2.Width := TotalWidth - ScrollBox2.Left;
    ScrollBox2.Height := ClientHeight - PanelButtons.Height;
    
    SpeedButtonOpenPdf3.Visible := False;
  end;
end;

procedure TFormMain.SpeedButtonThreeViewClick(Sender: TObject);

begin

ThreeViewMode := not ThreeViewMode;

SpeedButtonThreeView.Down := ThreeViewMode;

UpdateLayout;

end;

procedure TFormMain.UpdateLayout;

var

TotalWidth: Integer;

begin

if ThreeViewMode then

begin

// Three view mode: equal thirds

ScrollBox1.Align := alNone;

ScrollBox2.Align := alNone;

ScrollBox3.Align := alNone;

Splitter1.Visible := False;

Splitter2.Visible := False;

TotalWidth := ClientWidth;

ScrollBox1.Left := 0;

ScrollBox1.Width := TotalWidth div 3;

ScrollBox1.Height := ClientHeight - PanelButtons.Height;

ScrollBox2.Left := ScrollBox1.Width;

ScrollBox2.Width := TotalWidth div 3;

ScrollBox2.Height := ClientHeight - PanelButtons.Height;

ScrollBox3.Left := ScrollBox2.Left + ScrollBox2.Width;

ScrollBox3.Width := TotalWidth - ScrollBox3.Left;

ScrollBox3.Height := ClientHeight - PanelButtons.Height;

ScrollBox3.Visible := True;

SpeedButtonOpenPdf3.Visible := True;

end

else

begin

// Two view mode: equal halves

ScrollBox3.Visible := False;

Splitter2.Visible := False;

TotalWidth := ClientWidth;

ScrollBox1.Left := 0;

ScrollBox1.Width := TotalWidth div 2;

ScrollBox1.Height := ClientHeight - PanelButtons.Height;

ScrollBox2.Left := ScrollBox1.Width;

ScrollBox2.Width := TotalWidth - ScrollBox2.Left;

ScrollBox2.Height := ClientHeight - PanelButtons.Height;

SpeedButtonOpenPdf3.Visible := False;

end;

Opening PDFs in Each Panel

procedure TFormMain.OpenPdfFile(PdfComponent: TPdf;
  PdfViewComponent: TPdfView; const Title: string);
var
  Password: string;
begin
  if OpenDialog.Execute then
  begin
    PdfComponent.Active := False;
    PdfComponent.FileName := OpenDialog.FileName;
    PdfComponent.Password := '';
    
    try
      PdfViewComponent.Active := True;
    except
      on Error: EPdfError do
        if Error.Message = 'Password required or incorrect password' then
        begin
          if InputQuery('Enter Password', 'Password: ', Password) then
          begin
            PdfComponent.Password := Password;
            PdfViewComponent.Active := True;
          end
          else
            raise;
        end
        else
          raise;
    end;
    
    SetActivePdfView(PdfViewComponent);
    UpdateControlsState;
  end;
end;

procedure TFormMain.SpeedButtonOpenPdf1Click(Sender: TObject);
begin
  OpenPdfFile(Pdf1, PdfView1, 'PDF 1');
end;

procedure TFormMain.SpeedButtonOpenPdf2Click(Sender: TObject);
begin
  OpenPdfFile(Pdf2, PdfView2, 'PDF 2');
end;

procedure TFormMain.SpeedButtonOpenPdf3Click(Sender: TObject);
begin
  OpenPdfFile(Pdf3, PdfView3, 'PDF 3');
end;

procedure TFormMain.OpenPdfFile(PdfComponent: TPdf;

PdfViewComponent: TPdfView; const Title: string);

var

Password: string;

begin

if OpenDialog.Execute then

begin

PdfComponent.Active := False;

PdfComponent.FileName := OpenDialog.FileName;

PdfComponent.Password := '';

try

PdfViewComponent.Active := True;

except

on Error: EPdfError do

if Error.Message = 'Password required or incorrect password' then

begin

if InputQuery('Enter Password', 'Password: ', Password) then

begin

PdfComponent.Password := Password;

PdfViewComponent.Active := True;

end

else

raise;

end

else

raise;

end;

SetActivePdfView(PdfViewComponent);

UpdateControlsState;

end;

procedure TFormMain.SpeedButtonOpenPdf1Click(Sender: TObject);

begin

OpenPdfFile(Pdf1, PdfView1, 'PDF 1');

end;

procedure TFormMain.SpeedButtonOpenPdf2Click(Sender: TObject);

begin

OpenPdfFile(Pdf2, PdfView2, 'PDF 2');

end;

procedure TFormMain.SpeedButtonOpenPdf3Click(Sender: TObject);

begin

OpenPdfFile(Pdf3, PdfView3, 'PDF 3');

end;

Managing Active Panel

procedure TFormMain.SetActivePdfView(PdfView: TPdfView);
begin
  FActivePdfView := PdfView;
  
  // Visual feedback for active panel
  if PdfView = PdfView1 then
  begin
    ScrollBox1.Color := clHighlight;
    ScrollBox2.Color := clWindow;
    ScrollBox3.Color := clWindow;
  end
  else if PdfView = PdfView2 then
  begin
    ScrollBox1.Color := clWindow;
    ScrollBox2.Color := clHighlight;
    ScrollBox3.Color := clWindow;
  end
  else if PdfView = PdfView3 then
  begin
    ScrollBox1.Color := clWindow;
    ScrollBox2.Color := clWindow;
    ScrollBox3.Color := clHighlight;
  end;
  
  UpdateControlsState;
end;

procedure TFormMain.PdfView1Click(Sender: TObject);
begin
  SetActivePdfView(PdfView1);
end;

procedure TFormMain.PdfView2Click(Sender: TObject);
begin
  SetActivePdfView(PdfView2);
end;

procedure TFormMain.PdfView3Click(Sender: TObject);
begin
  SetActivePdfView(PdfView3);
end;

procedure TFormMain.SetActivePdfView(PdfView: TPdfView);

begin

FActivePdfView := PdfView;

// Visual feedback for active panel

if PdfView = PdfView1 then

begin

ScrollBox1.Color := clHighlight;

ScrollBox2.Color := clWindow;

ScrollBox3.Color := clWindow;

end

else if PdfView = PdfView2 then

begin

ScrollBox1.Color := clWindow;

ScrollBox2.Color := clHighlight;

ScrollBox3.Color := clWindow;

end

else if PdfView = PdfView3 then

begin

ScrollBox1.Color := clWindow;

ScrollBox2.Color := clWindow;

ScrollBox3.Color := clHighlight;

end;

UpdateControlsState;

end;

procedure TFormMain.PdfView1Click(Sender: TObject);

begin

SetActivePdfView(PdfView1);

end;

procedure TFormMain.PdfView2Click(Sender: TObject);

begin

SetActivePdfView(PdfView2);

end;

procedure TFormMain.PdfView3Click(Sender: TObject);

begin

SetActivePdfView(PdfView3);

end;

Applying Actions to Active or All Views

procedure TFormMain.ApplyToAllViews(const Operation: string);
begin
  if FAllViewsMode then
  begin
    // Apply to all active views
    if PdfView1.Active then
      ApplyOperation(PdfView1, Operation);
    if PdfView2.Active then
      ApplyOperation(PdfView2, Operation);
    if PdfView3.Active and ThreeViewMode then
      ApplyOperation(PdfView3, Operation);
  end
  else
  begin
    // Apply only to active view
    if Assigned(FActivePdfView) then
      ApplyOperation(FActivePdfView, Operation);
  end;
end;

procedure TFormMain.SpeedButtonZoomInClick(Sender: TObject);
begin
  ApplyToAllViews('ZoomIn');
end;

procedure TFormMain.SpeedButtonZoomOutClick(Sender: TObject);
begin
  ApplyToAllViews('ZoomOut');
end;

procedure TFormMain.ApplyToAllViews(const Operation: string);

begin

if FAllViewsMode then

begin

// Apply to all active views

if PdfView1.Active then

ApplyOperation(PdfView1, Operation);

if PdfView2.Active then

ApplyOperation(PdfView2, Operation);

if PdfView3.Active and ThreeViewMode then

ApplyOperation(PdfView3, Operation);

end

else

begin

// Apply only to active view

if Assigned(FActivePdfView) then

ApplyOperation(FActivePdfView, Operation);

end;

procedure TFormMain.SpeedButtonZoomInClick(Sender: TObject);

begin

ApplyToAllViews('ZoomIn');

end;

procedure TFormMain.SpeedButtonZoomOutClick(Sender: TObject);

begin

ApplyToAllViews('ZoomOut');

end;

Synchronized Page Navigation (Optional)

procedure TFormMain.SynchronizePages(SourceView: TPdfView);
var
  PageNum: Integer;
begin
  if not chkSynchronize.Checked then
    Exit;
    
  PageNum := SourceView.PageNumber;
  
  // Apply to other views
  if (SourceView &lt;&gt; PdfView1) and PdfView1.Active then
    if PageNum &lt;= PdfView1.PageCount then
      PdfView1.PageNumber := PageNum;
      
  if (SourceView &lt;&gt; PdfView2) and PdfView2.Active then
    if PageNum &lt;= PdfView2.PageCount then
      PdfView2.PageNumber := PageNum;
      
  if (SourceView &lt;&gt; PdfView3) and PdfView3.Active and ThreeViewMode then
    if PageNum &lt;= PdfView3.PageCount then
      PdfView3.PageNumber := PageNum;
end;

procedure TFormMain.SynchronizePages(SourceView: TPdfView);

var

PageNum: Integer;

begin

if not chkSynchronize.Checked then

Exit;

PageNum := SourceView.PageNumber;

// Apply to other views

if (SourceView <> PdfView1) and PdfView1.Active then

if PageNum <= PdfView1.PageCount then

PdfView1.PageNumber := PageNum;

if (SourceView <> PdfView2) and PdfView2.Active then

if PageNum <= PdfView2.PageCount then

PdfView2.PageNumber := PageNum;

if (SourceView <> PdfView3) and PdfView3.Active and ThreeViewMode then

if PageNum <= PdfView3.PageCount then

PdfView3.PageNumber := PageNum;

end;

Independent Zoom Controls

procedure TFormMain.DoZoom1;
begin
  if PdfView1.Active and (PdfView1.PageNumber &gt; 0) then
  begin
    case ComboBoxZoom.ItemIndex of
      0: PdfView1.Zoom := 0.5;
      1: PdfView1.Zoom := 0.75;
      2: PdfView1.Zoom := 1.0;
      3: PdfView1.Zoom := 1.5;
      4: PdfView1.Zoom := 2.0;
      5: PdfView1.Zoom := PdfView1.PageWidthZoom[PdfView1.PageNumber];
      6: PdfView1.Zoom := PdfView1.PageZoom[PdfView1.PageNumber];
    end;
  end;
end;

procedure TFormMain.DoZoom2;
begin
  if PdfView2.Active and (PdfView2.PageNumber &gt; 0) then
  begin
    // Same zoom logic for view 2
  end;
end;

procedure TFormMain.DoZoom3;
begin
  if PdfView3.Active and (PdfView3.PageNumber &gt; 0) then
  begin
    // Same zoom logic for view 3
  end;
end;

procedure TFormMain.DoZoom1;

begin

if PdfView1.Active and (PdfView1.PageNumber > 0) then

begin

case ComboBoxZoom.ItemIndex of

0: PdfView1.Zoom := 0.5;

1: PdfView1.Zoom := 0.75;

2: PdfView1.Zoom := 1.0;

3: PdfView1.Zoom := 1.5;

4: PdfView1.Zoom := 2.0;

5: PdfView1.Zoom := PdfView1.PageWidthZoom[PdfView1.PageNumber];

6: PdfView1.Zoom := PdfView1.PageZoom[PdfView1.PageNumber];

end;

procedure TFormMain.DoZoom2;

begin

if PdfView2.Active and (PdfView2.PageNumber > 0) then

begin

// Same zoom logic for view 2

end;

procedure TFormMain.DoZoom3;

begin

if PdfView3.Active and (PdfView3.PageNumber > 0) then

begin

// Same zoom logic for view 3

end;

Saving Current View as Image

procedure TFormMain.MenuItemSaveAsImage1Click(Sender: TObject);
begin
  SaveViewAsImage(PdfView1, Pdf1);
end;

procedure TFormMain.SaveViewAsImage(PdfView: TPdfView; Pdf: TPdf);
var
  Bitmap: TBitmap;
  JpegImage: TJpegImage;
begin
  if not PdfView.Active then
  begin
    ShowMessage('No document loaded in this view.');
    Exit;
  end;
  
  SavePictureDialog.FileName := 'page_' + IntToStr(PdfView.PageNumber) + '.jpg';
  
  if SavePictureDialog.Execute then
  begin
    Pdf.PageNumber := PdfView.PageNumber;
    
    Bitmap := Pdf.RenderPage(
      0, 0,
      Round(Pdf.PageWidth * 2),  // 2x resolution
      Round(Pdf.PageHeight * 2),
      PdfView.Rotation, [], clWhite
    );
    
    try
      JpegImage := TJpegImage.Create;
      try
        JpegImage.Assign(Bitmap);
        JpegImage.CompressionQuality := 90;
        JpegImage.SaveToFile(SavePictureDialog.FileName);
        ShowMessage('Image saved successfully.');
      finally
        JpegImage.Free;
      end;
    finally
      Bitmap.Free;
    end;
  end;
end;

procedure TFormMain.MenuItemSaveAsImage1Click(Sender: TObject);

begin

SaveViewAsImage(PdfView1, Pdf1);

end;

procedure TFormMain.SaveViewAsImage(PdfView: TPdfView; Pdf: TPdf);

var

Bitmap: TBitmap;

JpegImage: TJpegImage;

begin

if not PdfView.Active then

begin

ShowMessage('No document loaded in this view.');

Exit;

end;

SavePictureDialog.FileName := 'page_' + IntToStr(PdfView.PageNumber) + '.jpg';

if SavePictureDialog.Execute then

begin

Pdf.PageNumber := PdfView.PageNumber;

Bitmap := Pdf.RenderPage(

0, 0,

Round(Pdf.PageWidth * 2), // 2x resolution

Round(Pdf.PageHeight * 2),

PdfView.Rotation, [], clWhite

);

try

JpegImage := TJpegImage.Create;

try

JpegImage.Assign(Bitmap);

JpegImage.CompressionQuality := 90;

JpegImage.SaveToFile(SavePictureDialog.FileName);

ShowMessage('Image saved successfully.');

finally

JpegImage.Free;

end;

finally

Bitmap.Free;

end;

Text Selection in Each Panel

procedure TFormMain.PdfView1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  SetActivePdfView(PdfView1);
  
  if Button = mbLeft then
  begin
    Selecting := True;
    SelectionStart := PdfView1.CharacterIndexAtPos(X, Y, 5.0, 5.0);
    SelectionEnd := SelectionStart;
  end;
end;

procedure TFormMain.PdfView1MouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
  if Selecting then
  begin
    SelectionEnd := PdfView1.CharacterIndexAtPos(X, Y, 5.0, 5.0);
    PdfView1.Invalidate;
  end;
end;

procedure TFormMain.PdfView1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Selecting := False;
end;

procedure TFormMain.PdfView1MouseDown(Sender: TObject; Button: TMouseButton;

Shift: TShiftState; X, Y: Integer);

begin

SetActivePdfView(PdfView1);

if Button = mbLeft then

begin

Selecting := True;

SelectionStart := PdfView1.CharacterIndexAtPos(X, Y, 5.0, 5.0);

SelectionEnd := SelectionStart;

end;

procedure TFormMain.PdfView1MouseMove(Sender: TObject; Shift: TShiftState;

X, Y: Integer);

begin

if Selecting then

begin

SelectionEnd := PdfView1.CharacterIndexAtPos(X, Y, 5.0, 5.0);

PdfView1.Invalidate;

end;

procedure TFormMain.PdfView1MouseUp(Sender: TObject; Button: TMouseButton;

Shift: TShiftState; X, Y: Integer);

begin

Selecting := False;

end;

Use Cases

Document Revision Review – Compare original and revised documents
Quality Control – Verify production against templates
Legal Review – Compare contract versions
Translation Verification – View source and translated documents
Multi-language Documents – View same content in different languages

Conclusion

The Split View demo showcases the flexibility of PDFium VCL for building advanced document comparison tools. With support for multiple independent views, synchronized navigation, and per-panel controls, you can create professional comparison applications.

The ability to open different PDFs in each panel and compare them visually is invaluable for revision review and quality control workflows.

Build powerful document comparison tools with Delphi PDFium VCL Component.