Merging & Layouts

PDFium allows you to combine multiple documents into one or create “N-up” layouts (multiple pages per sheet) for printing.

Merging Documents

You can import pages from one document into another. This is done using the importPages or importPagesByIndex methods.

Basic Merge (Append All)

import { PDFium } from '@scaryterry/pdfium';

using pdfium = await PDFium.init();
using destDoc = await pdfium.createDocument(); // Or open an existing one
using sourceDoc = await pdfium.openDocument(sourceBytes);

// Append all pages from sourceDoc to the end of destDoc
destDoc.importPages(sourceDoc);

// Save the result
const mergedBytes = destDoc.save();

Partial Merge (Specific Pages)

You can specify a page range string (e.g., “1,3,5-7”) or an array of indices.

// Import pages 1, 3, and 5 through 7 (1-based index string)
destDoc.importPages(sourceDoc, {
  pageRange: '1,3,5-7',
  insertIndex: 0 // Insert at the beginning
});

// OR: Import by zero-based index array
destDoc.importPagesByIndex(sourceDoc, [0, 2, 4], 0);

Caution

Resources like fonts and images are copied into the destination document. Importing pages from many different source documents may result in a large file size due to duplicated resources.

N-Up Layouts (Printing)

“N-up” refers to placing multiple pages from a source document onto a single page in a new document. This is commonly used for printing handouts (e.g., 2 slides per page) or booklets.

Creating a 2-Up Document

using sourceDoc = await pdfium.openDocument(pdfBytes);

// Create a new document where every 2 source pages = 1 output page
// The output page size is A4 Landscape (842 x 595 points)
using nUpDoc = sourceDoc.createNUpDocument({
  outputWidth: 842,
  outputHeight: 595,
  pagesPerRow: 2,
  pagesPerColumn: 1
});

if (nUpDoc) {
  const bytes = nUpDoc.save();
  // nUpDoc is a completely new document instance
}

Common Layouts

Layout	Rows	Cols	Orientation
2-Up (Side-by-side)	1	2	Landscape
4-Up (Grid)	2	2	Portrait
6-Up (Handouts)	3	2	Portrait

Copying Viewer Preferences

When merging documents, you might want to preserve the viewer preferences (like “Two Page View” or “Print Scaling”) from the source.

// Copy preferences (PrintScaling, Duplex, NumCopies, etc.)
destDoc.copyViewerPreferences(sourceDoc);