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Alternate & Mix two PDFs

Pick the front and back scan files, interleave in one click, everything stays on your device.

How Alternate & Mix two PDFs works

PDF Alternate Mix interleaves pages from two PDFs in strict alternating order: page 1 from document A, page 1 from document B, page 2 from A, page 2 from B, and so on. The result is a single merged PDF with pages in the sequence A1, B1, A2, B2. This is the operation you need after scanning a double-sided document on a single-sided scanner, where you scan all fronts in one pass and all backs in a second pass. pdf-lib assembles the interleaved document inside your browser, and neither file is sent to a server.

The tool also supports an optional reverse mode for the second document. When a single-sided scanner produces the back pages in reverse order (because the stack is flipped), enabling this option mirrors the back page sequence before interleaving, so page 1 front correctly pairs with page 1 back. For documents with an odd total page count, any leftover page from the longer file is appended at the end.

Written by Bastien Sulyan

How to use Alternate & Mix two PDFs, step by step

  1. Upload the first PDF (front pages, or document A) in the first drop zone.
  2. Upload the second PDF (back pages, or document B) in the second drop zone.
  3. If the back pages came out in reverse order from a single-sided scanner, enable the reverse second document option.
  4. Click merge and download the interleaved PDF.

Common use cases

  • You scanned a double-sided contract on a single-sided scanner: all odd pages in one pass, all even pages in a second pass; combine them into the correct reading order.
  • Two separate chapter PDFs need to be interleaved so that chapter A and chapter B alternate page by page for a comparative reading layout.
  • A batch of invoices and their corresponding receipts were scanned separately; interleave them so each invoice is immediately followed by its receipt.
  • A lecturer provided slides and speaker notes as two separate PDFs; merge them in alternating order for a combined study document.

Frequently asked questions

What does the reverse second document option do?

When you scan a stack of double-sided pages on a single-sided scanner by flipping the whole stack for the second pass, the back pages come out in reverse order (last page first). Enabling this option reverses document B before interleaving, so back page 1 ends up correctly placed after front page 1.

What happens if the two PDFs have different numbers of pages?

The tool interleaves as many paired pages as the shorter document provides. Any remaining pages from the longer document are appended in order at the end of the merged PDF.

Are my two PDF files sent to a server to be merged?

No. Both files are loaded into your browser's local memory and pdf-lib interleaves their pages there. The combined PDF is assembled in that same local memory and offered as a direct download to your device.

Can I interleave more than two PDFs at once?

This tool is designed for exactly two source documents. For more complex page ordering across multiple files, use the PDF Organise tool after merging, where you can drag individual page thumbnails into any sequence you need.

Does the tool preserve bookmarks, form fields and annotations from both source PDFs?

Page content, text layers and embedded images are preserved. Complex document-level metadata such as bookmarks and AcroForm fields may not survive the interleave operation intact, depending on how they reference page indices in the source files.