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Technical Article

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

· PDF Programming

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

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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

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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;

Opening PDFs in Each Panel

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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;

Managing Active Panel

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

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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;

Synchronized Page Navigation (Optional)

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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

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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;
  end;
end;
 
procedure TFormMain.DoZoom2;
begin
  if PdfView2.Active and (PdfView2.PageNumber > 0) then
  begin
    // Same zoom logic for view 2
  end;
end;
 
procedure TFormMain.DoZoom3;
begin
  if PdfView3.Active and (PdfView3.PageNumber > 0) then
  begin
    // Same zoom logic for view 3
  end;
end;

Saving Current View as Image

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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;

Text Selection in Each Panel

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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;

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.