Interleave PDFs — Alternate Pages from 2 PDFs Free Online in 4 Steps

🔀The Double-Sided Scan Problem — And Why Interleaving Solves It

If you've ever scanned a double-sided document on a flatbed scanner — not an ADF — you already know the problem. You scan all the odd-numbered pages (front sides) first, getting PDF A: pages 1, 3, 5, 7… Then you flip the stack over and scan all the even-numbered pages, getting PDF B: pages 8, 6, 4, 2… in reverse order. Now you have two separate PDFs and need to weave them together into the correct reading order.

This tool solves exactly that. It alternates pages from PDF A and PDF B — A1, B1, A2, B2, A3, B3 — to produce a single correctly ordered document. The "Reverse PDF B" checkbox handles the flipped stack scenario automatically, so you don't have to manually reverse anything before uploading. According to the Library of Congress digital format notes, PDF is the dominant format for digitized document archives — and interleaving is one of the most common operations archivists and document scanners need to perform regularly.

Beyond scanning, interleaving is useful for combining any two documents where the pages naturally pair: front and back of a form, two versions of a bilingual document alternating by page, presenter notes interleaved with slide pages, question sheets alternated with answer sheets. The pattern is the same regardless of the use case — and this tool handles it in your browser using pdf-lib, with no files sent anywhere.

📖How to Interleave Two PDFs — Step by Step

💡When to Use the "Reverse PDF B" Option

This is the option that confuses people most, so I'll be specific about when it's needed and when it isn't.

• Use it when: You scanned a physical document stack face-up on a flatbed. First pass: pages 1, 3, 5, 7 (PDF A). Second pass: you flipped the entire stack over without re-sorting, so the scanner captured pages 8, 6, 4, 2 — in reverse order. The Reverse B option fixes this automatically, turning that PDF B into 2, 4, 6, 8 before weaving.
• Don't use it when: Your scanner has an automatic document feeder (ADF) with duplex capability — most modern office scanners. These handle the page ordering themselves, so PDF B is already in the correct order. Or if you manually re-sorted the back pages into the correct order before scanning.
• How to check: Open PDF B and look at what's on the first and last pages. If the last page of your document appears first in PDF B, you need the Reverse option. If the first even-numbered page appears first, you don't.

The live preview diagram in the tool makes this easy to verify — it shows you exactly which page from which document lands in each position, so you can confirm the order is correct before committing to the download.

🏆Compared to Other Interleave Methods

The command-line tool pdftk (PDF Toolkit) is the classic technical solution for interleaving — its shuffle command does this in one line. But it requires installing software and knowing the syntax. This tool does the same thing in a browser with a visual preview, no installation, and no command line.

❓Frequently Asked Questions

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