Merging Multiple PDF Files into One Document with PDFium VCL

Combining multiple PDF documents into a single file is a common requirement in document management workflows. The Merge PDF demo shows how to merge any number of PDF files into one using PDFium VCL in Delphi.

Overview

This demo provides a simple interface for selecting multiple PDF files, reordering them, and merging them into a single output PDF. It preserves all content from the source documents including text, images, and formatting.

Key Features

Multiple File Selection – Add multiple PDFs at once
Reorder Files – Move files up or down in the merge order
Remove Files – Remove unwanted files from the list
Progress Tracking – Visual progress during merge operation
Preserve Content – All pages merged with original quality

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.

The Merge Process

The core merge functionality uses the ImportPages method:

procedure TFormMain.ButtonMergeClick(Sender: TObject);
var
  Page, I: Integer;
begin
  ProgressBar.Position := 0;
  ProgressBar.Enabled := True;
  Screen.Cursor := crHourGlass;
  
  try
    // Create new empty document for merged result
    PdfNew.CreateDocument;
    
    Page := 1;
    ProgressBar.Max := ListBox.Items.Count;
    
    // Process each source PDF
    for I := 0 to ListBox.Items.Count - 1 do
    begin
      // Load source PDF
      Pdf.FileName := ListBox.Items[I];
      Pdf.Active := True;
      
      // Import all pages from source
      PdfNew.ImportPages(Pdf, 
        IntToStr(1) + '-' + IntToStr(Pdf.PageCount), Page);
        
      // Track page position for next document
      Inc(Page, Pdf.PageCount);
      
      // Close source document
      Pdf.Active := False;
      
      ProgressBar.Position := I + 1;
    end;
    
    // Save merged document
    PdfNew.SaveAs(ExtractFilePath(Application.ExeName) + 'Merged.pdf');
    PdfNew.Active := False;
    
    MessageDlg('Merge completed', mtInformation, [mbOK], 0);
    
  finally
    Screen.Cursor := crDefault;
    ProgressBar.Position := 0;
    ProgressBar.Enabled := False;
    Pdf.Active := False;
    PdfNew.Active := False;
  end;
end;

procedure TFormMain.ButtonMergeClick(Sender: TObject);

var

Page, I: Integer;

begin

ProgressBar.Position := 0;

ProgressBar.Enabled := True;

Screen.Cursor := crHourGlass;

try

// Create new empty document for merged result

PdfNew.CreateDocument;

Page := 1;

ProgressBar.Max := ListBox.Items.Count;

// Process each source PDF

for I := 0 to ListBox.Items.Count - 1 do

begin

// Load source PDF

Pdf.FileName := ListBox.Items[I];

Pdf.Active := True;

// Import all pages from source

PdfNew.ImportPages(Pdf,

IntToStr(1) + '-' + IntToStr(Pdf.PageCount), Page);

// Track page position for next document

Inc(Page, Pdf.PageCount);

// Close source document

Pdf.Active := False;

ProgressBar.Position := I + 1;

end;

// Save merged document

PdfNew.SaveAs(ExtractFilePath(Application.ExeName) + 'Merged.pdf');

PdfNew.Active := False;

MessageDlg('Merge completed', mtInformation, [mbOK], 0);

finally

Screen.Cursor := crDefault;

ProgressBar.Position := 0;

ProgressBar.Enabled := False;

Pdf.Active := False;

PdfNew.Active := False;

end;

Understanding ImportPages

The ImportPages method is the key to PDF merging:

function TPdf.ImportPages(
  Pdf: TPdf;           // Source PDF document
  const Range: string; // Page range (e.g., "1-5", "1,3,5", "1-")
  PageNumber: Integer  // Insert position in destination
): Boolean;

function TPdf.ImportPages(

Pdf: TPdf; // Source PDF document

const Range: string; // Page range (e.g., "1-5", "1,3,5", "1-")

PageNumber: Integer // Insert position in destination

): Boolean;

Page range formats supported:

"1-5" – Pages 1 through 5
"1,3,5" – Pages 1, 3, and 5
"1-" – Page 1 to end
"-5" – First page to page 5

Adding Files to the List

procedure TFormMain.ButtonAddFilesClick(Sender: TObject);
var
  I: Integer;
begin
  if OpenDialog.Execute then
  begin
    for I := 0 to OpenDialog.Files.Count - 1 do
      ListBox.AddItem(OpenDialog.Files[I], nil);
      
    SelectItem(ListBox.Items.Count - 1);
    UpdateButtons;
  end;
end;

procedure TFormMain.ButtonAddFilesClick(Sender: TObject);

var

I: Integer;

begin

if OpenDialog.Execute then

begin

for I := 0 to OpenDialog.Files.Count - 1 do

ListBox.AddItem(OpenDialog.Files[I], nil);

SelectItem(ListBox.Items.Count - 1);

UpdateButtons;

end;

Reordering Files

procedure TFormMain.ButtonMoveUpClick(Sender: TObject);
var
  ItemIndex: Integer;
begin
  ItemIndex := ListBox.ItemIndex;
  ListBox.Items.Move(ItemIndex, ItemIndex - 1);
  SelectItem(ItemIndex - 1);
  UpdateButtons;
end;

procedure TFormMain.ButtonMoveDownClick(Sender: TObject);
var
  ItemIndex: Integer;
begin
  ItemIndex := ListBox.ItemIndex;
  ListBox.Items.Move(ItemIndex, ItemIndex + 1);
  SelectItem(ItemIndex + 1);
  UpdateButtons;
end;

procedure TFormMain.ButtonMoveUpClick(Sender: TObject);

var

ItemIndex: Integer;

begin

ItemIndex := ListBox.ItemIndex;

ListBox.Items.Move(ItemIndex, ItemIndex - 1);

SelectItem(ItemIndex - 1);

UpdateButtons;

end;

procedure TFormMain.ButtonMoveDownClick(Sender: TObject);

var

ItemIndex: Integer;

begin

ItemIndex := ListBox.ItemIndex;

ListBox.Items.Move(ItemIndex, ItemIndex + 1);

SelectItem(ItemIndex + 1);

UpdateButtons;

end;

Removing Files

procedure TFormMain.ButtonDeleteClick(Sender: TObject);
var
  ItemIndex, ItemCount: Integer;
begin
  ItemIndex := ListBox.ItemIndex;
  
  if MessageDlg('Delete ' + ListBox.Items[ItemIndex] + '?',
    mtConfirmation, [mbYes, mbNo], 0) = mrYes then
  begin
    ListBox.Items.Delete(ItemIndex);
    ItemCount := ListBox.Items.Count;
    
    // Select appropriate item after deletion
    if ItemIndex &lt; ItemCount then
      SelectItem(ItemIndex)
    else if ItemCount &gt; 0 then
      SelectItem(ItemIndex - 1);
      
    UpdateButtons;
  end;
end;

procedure TFormMain.ButtonDeleteClick(Sender: TObject);

var

ItemIndex, ItemCount: Integer;

begin

ItemIndex := ListBox.ItemIndex;

if MessageDlg('Delete ' + ListBox.Items[ItemIndex] + '?',

mtConfirmation, [mbYes, mbNo], 0) = mrYes then

begin

ListBox.Items.Delete(ItemIndex);

ItemCount := ListBox.Items.Count;

// Select appropriate item after deletion

if ItemIndex < ItemCount then

SelectItem(ItemIndex)

else if ItemCount > 0 then

SelectItem(ItemIndex - 1);

UpdateButtons;

end;

Updating UI State

procedure TFormMain.UpdateButtons;
var
  ItemIndex, ItemCount: Integer;
begin
  ItemIndex := ListBox.ItemIndex;
  ItemCount := ListBox.Items.Count;
  
  ButtonAddFiles.Enabled := True;
  ButtonDelete.Enabled := ItemIndex &lt;&gt; -1;
  ButtonMoveUp.Enabled := (ItemIndex &lt;&gt; -1) and (ItemIndex &gt; 0);
  ButtonMoveDown.Enabled := (ItemIndex &lt;&gt; -1) and (ItemIndex &lt; ItemCount - 1);
  ButtonMerge.Enabled := ItemCount &gt; 1; // Need at least 2 files to merge
end;

procedure TFormMain.UpdateButtons;

var

ItemIndex, ItemCount: Integer;

begin

ItemIndex := ListBox.ItemIndex;

ItemCount := ListBox.Items.Count;

ButtonAddFiles.Enabled := True;

ButtonDelete.Enabled := ItemIndex <> -1;

ButtonMoveUp.Enabled := (ItemIndex <> -1) and (ItemIndex > 0);

ButtonMoveDown.Enabled := (ItemIndex <> -1) and (ItemIndex < ItemCount - 1);

ButtonMerge.Enabled := ItemCount > 1; // Need at least 2 files to merge

end;

Advanced: Merging Specific Pages

You can merge only specific pages from source documents:

procedure MergeSpecificPages;
var
  PdfSource, PdfDest: TPdf;
begin
  PdfSource := TPdf.Create(nil);
  PdfDest := TPdf.Create(nil);
  try
    PdfDest.CreateDocument;
    
    // Import pages 1-3 from first document
    PdfSource.FileName := 'document1.pdf';
    PdfSource.Active := True;
    PdfDest.ImportPages(PdfSource, '1-3', 1);
    PdfSource.Active := False;
    
    // Import only page 5 from second document
    PdfSource.FileName := 'document2.pdf';
    PdfSource.Active := True;
    PdfDest.ImportPages(PdfSource, '5', 4);
    PdfSource.Active := False;
    
    // Import last 2 pages from third document
    PdfSource.FileName := 'document3.pdf';
    PdfSource.Active := True;
    PdfDest.ImportPages(PdfSource, 
      IntToStr(PdfSource.PageCount - 1) + '-' + IntToStr(PdfSource.PageCount), 5);
    PdfSource.Active := False;
    
    PdfDest.SaveAs('merged_custom.pdf');
    
  finally
    PdfDest.Active := False;
    PdfSource.Free;
    PdfDest.Free;
  end;
end;

procedure MergeSpecificPages;

var

PdfSource, PdfDest: TPdf;

begin

PdfSource := TPdf.Create(nil);

PdfDest := TPdf.Create(nil);

try

PdfDest.CreateDocument;

// Import pages 1-3 from first document

PdfSource.FileName := 'document1.pdf';

PdfSource.Active := True;

PdfDest.ImportPages(PdfSource, '1-3', 1);

PdfSource.Active := False;

// Import only page 5 from second document

PdfSource.FileName := 'document2.pdf';

PdfSource.Active := True;

PdfDest.ImportPages(PdfSource, '5', 4);

PdfSource.Active := False;

// Import last 2 pages from third document

PdfSource.FileName := 'document3.pdf';

PdfSource.Active := True;

PdfDest.ImportPages(PdfSource,

IntToStr(PdfSource.PageCount - 1) + '-' + IntToStr(PdfSource.PageCount), 5);

PdfSource.Active := False;

PdfDest.SaveAs('merged_custom.pdf');

finally

PdfDest.Active := False;

PdfSource.Free;

PdfDest.Free;

end;

Importing Viewer Preferences

Copy viewer preferences from another document:

procedure CopyViewerPreferences;
var
  PdfSource, PdfDest: TPdf;
begin
  PdfSource := TPdf.Create(nil);
  PdfDest := TPdf.Create(nil);
  try
    PdfSource.FileName := 'template.pdf';
    PdfSource.Active := True;
    
    PdfDest.FileName := 'output.pdf';
    PdfDest.Active := True;
    
    // Copy viewer preferences (zoom, page layout, etc.)
    PdfDest.ImportPreferences(PdfSource);
    
    PdfDest.SaveAs('output.pdf');
    
  finally
    PdfSource.Active := False;
    PdfDest.Active := False;
    PdfSource.Free;
    PdfDest.Free;
  end;
end;

procedure CopyViewerPreferences;

var

PdfSource, PdfDest: TPdf;

begin

PdfSource := TPdf.Create(nil);

PdfDest := TPdf.Create(nil);

try

PdfSource.FileName := 'template.pdf';

PdfSource.Active := True;

PdfDest.FileName := 'output.pdf';

PdfDest.Active := True;

// Copy viewer preferences (zoom, page layout, etc.)

PdfDest.ImportPreferences(PdfSource);

PdfDest.SaveAs('output.pdf');

finally

PdfSource.Active := False;

PdfDest.Active := False;

PdfSource.Free;

PdfDest.Free;

end;

Use Cases

Report Compilation – Combine multiple report sections into one document
Invoice Batching – Merge daily invoices into monthly batches
Document Assembly – Build complete documents from templates and content
Archive Creation – Combine related documents for archival

Conclusion

The Merge PDF demo shows how straightforward it is to combine PDF documents with PDFium VCL. The ImportPages method handles all the complexity of preserving document structure and content.

Whether you’re building a simple document combiner or a complex document assembly system, PDFium VCL provides the tools you need.

Get started with PDFium VCL Component from loslab.com and streamline your PDF workflows.