Alternate Merge (Duplex) - Free Online Browser Tool

Merge two PDFs by alternating their pages (1a, 1b, 2a, 2b...).

Duplex scanning often leaves you with one file containing all the fronts and another containing all the backs. This tool interleaves them automatically — front page 1, back page 1, front page 2, back page 2 — which is exactly the order a duplex scanner or finisher expects.

This alternate merge (duplex) utility runs completely inside your web browser. Your document is opened locally and processed on your own device using JavaScript and WebAssembly, so your file is never uploaded to any server. There is no signup, no watermark and no limit on how many documents you can handle.

  • 100% client-side - files never leave your device
  • Instant processing, no upload wait
  • Works on desktop, tablet and mobile
  • Free forever, no account required

Common next steps: Booklet Maker, Add Extra Margins, Reverse Page Order, Extract Odd/Even Pages.

How to Use the Alternate Merge (Duplex) Tool

Four steps, and the whole thing runs locally on your own device.

  1. Add the two documents, fronts first

    Drop in the file holding all odd/front pages and the file holding all even/back pages. Load the front-side document first so the alternation starts correctly.

  2. Confirm the pairing

    The panel reports the page count of each file. If one side has more pages than the other the extra pages are simply appended at the end rather than dropped.

  3. Alternate and build

    Press Alternate Merge. Output is 1a, 1b, 2a, 2b and so on, ready to be reprinted duplex on a long-edge or short-edge machine.

  4. Save the interleaved PDF

    Download the result and feed it straight back to your printer or scanner’s duplex tray.

Frequently Asked Questions

What if the two files have different page counts?

Alternation continues until the shorter file runs out, then the remaining pages of the longer file are appended in their original order. Nothing is discarded.

Can I alternate more than two files?

This tool pairs exactly two documents, which covers the standard duplex case. For three or more interleaved scans, use Merge PDFs and order the chips manually.

Which binding edge should I choose when printing the result?

Use long-edge binding for books and reports flipped like a calendar, and short-edge for anything flipped like a notepad. The tool does not encode this — it is a printer setting.

Does the original scan quality survive?

Yes. Pages are copied, not re-imaged, so resolution and compression artefacts stay exactly as the scanner produced them.

My scanner already produced one interleaved file. Do I need this?

No. Use Merge PDF instead — this tool is for the case where your scanner gave you separate front and back files.