Combine two PDFs by alternating pages: 1A, 1B, 2A, 2B… Perfect for duplex scanning.
Drop your PDF here
or click to select
Drop your PDF here
or click to select
FusionPDF interleaves two PDFs by alternating pages: page 1 from PDF A, page 1 from PDF B, page 2 from A, page 2 from B, and so on. Used for combining front and back pages from double-sided scans done in two passes. Runs entirely in your browser using pdf-lib — no upload.
Drop your first PDF into the "1st PDF" zone and your second PDF into the "2nd PDF" zone. The tool shows the file name, size, and page count for each after loading.
An optional checkbox lets you reverse the page order of the second PDF before interleaving. This is useful for duplex scan correction, where the back pages come out of the scanner in reverse order.
Click "Interleave and download." pdf-lib copies pages from both documents alternately into a new PDF: A1, B1, A2, B2, A3, B3, and so on. If one PDF has more pages than the other, the extra pages from the longer file are appended at the end. The combined PDF downloads immediately.
Many document scanners, especially older office models, scan one side at a time. The standard duplex-scan workflow on a single-sided scanner produces two separate PDFs.
You place the stack face-down and scan all front pages. This gives you PDF A: pages 1, 3, 5, 7 (the odd pages in reading order). You then flip the physical stack and scan the back faces. The scanner sees the back of the last front page first, so this gives you PDF B: pages 8, 6, 4, 2 — in reverse order.
To reassemble the document, you reverse PDF B first (using the Reverse PDF tool), bringing it to order 2, 4, 6, 8. Then you interleave PDF A and the corrected PDF B. The result is the correct reading sequence: 1, 2, 3, 4, 5, 6, 7, 8.
This is by far the most common use case for interleaving. The "Reverse 2nd PDF" checkbox in this tool is there precisely for this workflow, letting you skip the separate reverse step.
Both tools combine multiple PDFs into one, but the page ordering is completely different.
If you are unsure which to use, think about what the two PDFs represent. Two separate chapters to combine in order: merge. Front faces and back faces of the same document: interleave.
The tool handles unequal page counts automatically. After alternating pages up to the length of the shorter PDF, any remaining pages from the longer PDF are appended sequentially at the end.
For example: PDF A has 6 pages, PDF B has 4 pages. The output is: A1, B1, A2, B2, A3, B3, A4, B4, A5, A6. The two extra pages from A follow the end of the interleaved section.
This means you don't need to pad the shorter PDF to match lengths. The tool handles the mismatch without any extra steps from you.
Double-sided scanned documents often contain sensitive material: official forms, signed contracts, identification documents, financial records. Both PDFs you load into this tool are processed entirely in your browser using pdf-lib. Neither file is transmitted to any server at any point. No data is stored after the tab is closed.