Merge PDF Alternating Pages

Interleave two PDFs, taking pages alternately from each — perfect for combining separate front and back scans.

The fix for a duplex scan done in two passes

Here is a specific, maddening situation this solves. You scan a stack of double-sided pages, but your scanner only does one side, so you run all the fronts, then all the backs. Now you have two PDFs — odd pages in one, even pages in the other — and you need them woven together in the right order. Alternating merge does exactly that, taking one page from each file in turn.

Provide the two PDFs and it interleaves them into a single, correctly ordered document.

Getting the second file the right way round

There is one wrinkle worth knowing. If you scanned the backs by flipping the whole stack, the second PDF is in reverse order, and interleaving it as-is puts every back page with the wrong front. In that case, reversing the second file first sets it straight. Once both files are in the right sequence, the alternating merge reassembles the document exactly as if you had a duplex scanner all along.

How to merge two PDFs with alternating pages

Add the two PDFs — for example the fronts and the backs of a stack.
Interleave them so pages alternate: front, back, front, back.
Export the correctly collated document.

Rescue a single-sided scanner

If your scanner only does one side, you end up with two files — all the odd pages, then all the even pages (often in reverse). Alternating merge weaves them back into the correct order, so a double-sided document reads properly.

If the second file was scanned back-to-front, reverse it first, then interleave.