About Alternate Merge PDF
A sheet feeder that reads only one side of the paper can still copy a double-sided document in two passes: the fronts first, then the stack turned over for the backs. That leaves two PDFs, one with the odd-numbered pages of the original and one with the even ones, and this tool weaves them back into a single document in reading order.
The pattern is strictly one from each: page 1 of file A, page 1 of file B, page 2 of A, page 2 of B, and onward (A1, B1, A2, B2 …). If one file runs longer, nothing is thrown away; its leftover pages are appended after the last complete pair.
Turning a stack over normally reverses it, so the back of the final sheet is scanned first. That is what the Second file is in reverse order option is for. It is off by default; switched on, it reads the second file from its last page to its first while interleaving, with no separate reversing step.
The pages themselves pass through untouched. They are copied into the new PDF without being redrawn or recompressed, which means scans keep their resolution, any text layer stays searchable, and the combined file weighs about as much as the two inputs together.
How to use Alternate Merge PDF
- Load the two scansAdd the PDF of front sides as the first file and the PDF of back sides as the second.
- Tell it how the backs runIf you flipped the whole stack before the second pass, the backs file most likely starts with the last sheet, so tick Second file is in reverse order. Leave it unticked when the backs already run from the first sheet to the last.
- Combine and spot-checkStart the merge, download the result and compare its first and last two pages: each front should be followed directly by its own back.
When people use it
A lease from a one-sided office feeder
The copier at work scans one side only. Feed the 12-sheet lease face up, flip the pile, feed it again, and the two 12-page PDFs come together as the 24-page lease in its original sequence.
Family letters flipped for the second pass
Turning a pile of handwritten letters over for the reverse sides put the last letter’s back at the top of the second PDF. With the reverse option ticked, every back still lands right after the matching front.
One-sided attachments at the end of a packet
A 10-sheet application is double-sided apart from three one-sided attachments at the back, left out of the second pass. With 10 fronts and 7 backs, the three extra fronts follow the last pair.
Odd pages first, even pages second
The first file supplies pages 1, 3, 5 and so on of the finished document; the second supplies 2, 4, 6. Naming the scans fronts.pdf and backs.pdf as soon as they come off the scanner keeps the two roles from getting mixed up.
If your scanner wrote both passes into one long PDF rather than two, separate them first. In Split PDF, By ranges mode with something like 1-12, 13-24 produces the two halves, which you can then add here.
Reading the result when something is off
| What you see | Likely cause | Fix |
|---|---|---|
| Page 2 is the back of the last sheet | The backs were scanned last sheet first | Run again with Second file is in reverse order ticked |
| The document opens on a back | The two files went in with their roles swapped | Run again with the fronts as the first file |
| Pairs line up for a while, then drift by one page | A sheet was skipped or double-fed in one pass | Rescan that pass, or move the stray pages with Rearrange PDF |
| Every second page is upside down | The stack was turned over top to bottom | Turn the even pages 180° in Rotate PDF |
Upside-down backs and empty reverse sides
Flipping the stack sideways, the way you turn a book page, usually keeps the backs upright; flipping it top to bottom stands them on their heads. Interleaving keeps each page as it was scanned, so fix the even pages afterwards with the even-pages button in Rotate PDF rather than scanning again.
Sheets with nothing on the back leave empty pages behind; Remove blank pages finds them and lets you confirm each one.
Size, properties and other limits
- Size. Since the output is roughly the two scans added together, Compress PDF may be needed before emailing it.
- Document properties. Treat the output as a new document; if it needs a proper title or author, set them with Edit PDF metadata.
- Two files per run. The tool pairs exactly two PDFs. To put whole documents one after another instead of shuffling their pages, use Merge PDF.
- Locked scans. Encrypted files won’t open here until Unlock PDF removes the password, which you must know.
- Memory. Both files sit in the tab’s memory during the job, and a desktop or laptop has far more to spare than a phone, so give very long, high-resolution scans to a computer.
The two files are combined on your device and never uploaded; here is how to confirm that.