This solves one specific and very annoying problem: you have a double-sided document and a scanner that only reads one side. You scan all the fronts, turn the stack over, scan all the backs, and end up with two PDFs that need to be zipped together like a deck of cards.
How the pages combine
One page is taken from each document in turn. Page 1 of the first file, then page 1 of the second, then page 2 of the first, page 2 of the second, and so on. For a fronts-and-backs scan that reassembles the original document exactly.
Order matters completely: the first file in the list supplies the odd pages. Put the backs scan first and every sheet comes out with its reverse side before its front. Use the arrows to check before combining.
The reversed-backs problem
Here is the trap. When you turn a stack over to scan the other side, the sheets feed in the opposite order — so the backs scan is usually reversed relative to the fronts. Interleaving directly then pairs the front of sheet one with the back of the last sheet.
The fix is to reverse the backs document first, then interleave. If your result has the right pages in obviously wrong pairings, this is why. Run the backs file through the page reverser and try again.
Uneven documents
- The files do not have to be the same length. When one runs out, remaining pages from the longer document are appended in order.
- That handles the common case where the last sheet is single-sided, and the backs scan is one page shorter.
- A blank final page in the backs scan is worth deleting first, since scanners often capture the empty reverse of the last sheet.
- More than two documents works too — a page is taken from each in rotation.
Lossless, like the other page operations
Pages are copied from both sources into a new document without being re-rendered. Scanned images keep their original bytes, so nothing softens, and any text layer stays selectable. The combined file is roughly the sum of the two inputs.
Everything runs in this tab. Double-sided documents being scanned this way are usually contracts, forms or records, and none of it is uploaded anywhere.
Test with a few sheets first
Before scanning a hundred double-sided pages, do four and interleave them. That tells you whether your scanner reverses the backs, whether the pairings come out right, and whether you need to reverse one file first — all in under a minute. Discovering it afterwards means scanning everything again.