Not every scanner supports automatic duplex (double-sided) scanning. The common workaround — scan every front side into one file, flip the whole stack, then scan every back side into a second file — works, but leaves you with two separate PDFs that need to be woven back together in the correct front-back-front-back reading order.

Why you can’t just merge them

A plain merge puts file B’s pages after file A’s, in a block — page 1 of the fronts, page 2 of the fronts, … then page 1 of the backs, page 2 of the backs. That’s not a readable document; it’s two documents stapled together in the wrong order.

The fix

Alternate & Mix takes exactly this case: upload your “fronts” file as the first PDF and your “backs” file as the second, and it interleaves them automatically — front 1, back 1, front 2, back 2, and so on — producing a single file in correct reading order.

  • If one file has more pages than the other (a common scanning mistake), the extra pages are added at the end rather than causing an error.
  • Everything runs in your browser, so scanned contracts, IDs, or other sensitive paperwork never get uploaded anywhere.

Check both files for stray pages before you mix

Do this before you interleave, not after: open each file and flip through the thumbnails looking for a blank separator sheet, a duplicate scan, or a page that jammed and got scanned twice. A stray page anywhere in the middle of either file shifts every page after it by one position, so every pair from that point on comes out front-with-front or back-with-back instead of front-with-back. Trimming the merged file afterward only fixes a stray page sitting right at the very end — it can’t undo the mismatch for everything that came before it, because by then the interleaving has already locked in the wrong pairing. If you spot a blank page in one of the source files, run that file through Remove Blank Pages first, then mix.

My backs came out in reverse order — what now?

Plenty of auto-feed scanners flip the whole paper stack over before scanning the second side, so the backs file comes out last-page-first (back of the last sheet, then the second-to-last, and so on) instead of in the same order as the fronts. Alternate & Mix doesn’t have a built-in option to flip a file’s order for you — it just interleaves both files exactly as uploaded, A1, B1, A2, B2. So if your backs are reversed, fix that first: open the backs file in Combine & Reorder and drag its page thumbnails into the opposite order, download that, and then upload it as the second file in Alternate & Mix.

Scan a 2-3 page test batch before committing to the full document — if your scanner’s software has a setting to scan backs in reverse order to begin with, matching how the stack naturally flips, turning it on saves you this extra reordering step every time.

After interleaving

If the finished, merged file has one extra page tacked on the very end — a trailing blank separator sheet, or a full duplicate scan that got appended after everything else — run it through Delete PDF Pages to drop just that page. No need to redo the interleaving from scratch. For anything other than a trailing page, go back and fix the source file instead, per the previous section.