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Technical Article

Merging Multiple PDF Files into One Document with PDFium VCL

· PDF Programming

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:

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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;

Understanding ImportPages

The ImportPages method is the key to PDF merging:

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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

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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;

Reordering Files

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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

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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;
end;

Updating UI State

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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:

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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;

Importing Viewer Preferences

Copy viewer preferences from another document:

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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;

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.