Skip to content
Initurn

Interleave two PDFs

Files never leave your device. All processing happens in your browser.

How to convert Two PDFs to Interleaved

  1. 1

    Add the fronts scan first, then the backs scan.

  2. 2

    Check the order — the first file supplies page 1.

  3. 3

    Combine into one correctly sequenced document.

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.

Questions about Two PDFs to Interleaved

What does interleaving actually do?

It takes one page from each document in turn — page 1 of the first, page 1 of the second, page 2 of the first, and so on. For a fronts scan and a backs scan, that rebuilds the original double-sided document.

Which file should go first?

The fronts scan, because the first file supplies the odd-numbered pages. Putting the backs first gives you every sheet with its reverse before its front.

My pages are paired with the wrong backs. What went wrong?

Turning the stack over makes the backs feed in reverse order, so that scan is usually backwards relative to the fronts. Reverse the backs document first, then interleave.

Do both files need the same number of pages?

No. When one runs out, the remaining pages of the longer file are appended in order, which handles a final sheet that was printed on one side only.

Can I interleave more than two documents?

Yes. A page is taken from each in rotation, in the order they appear in the list.

Is quality affected?

No. Pages are copied rather than re-rendered, so scanned images keep their exact original bytes and any text layer stays selectable.

My backs scan has a blank last page.

Scanners often capture the empty reverse of the final sheet. Delete that page first, or delete it from the combined result afterwards.

Related converters