How to scan double-sided documents without a duplex scanner
Updated · 4 min read
Most home and office scanners with a document feeder only scan one side of the paper. The usual workaround — scan the fronts, turn the stack over, scan the backs — leaves you with two PDFs and the backs in reverse order. Here's how to scan both sides and end up with one document that reads front 1, back 1, front 2, back 2.
Why the backs come out backwards
Picture a five-sheet stack. You scan the fronts from the top: sheet 1, 2, 3, 4, 5. Then you flip the whole stack over to scan the other side. Now sheet 5 is on top, so the scanner reads the back of sheet 5 first and the back of sheet 1 last. The second PDF is in reverse order.
A normal merge just puts one file after the other, which gives you all the fronts and then all the backs, backwards. What you need instead is to interleave the two files — alternate their pages — while reading the backs from last to first.
Step by step
- Scan all the fronts into one PDF. Keep the stack in order.
- Turn the stack over and scan all the backs into a second PDF. Don't re-sort the pages.
- Open Interleave PDF and add the fronts file first, then the backs file. Drag them to swap if needed.
- Leave Backs are in reverse order switched on — that's the flipped-stack case.
- Click Interleave and download the combined document.
Interleave runs in your browser; the scans aren't uploaded.
The result alternates the two files: the first page of the fronts, then the matching back, and so on. Pages are copied exactly as they were scanned — nothing is re-compressed — so the combined file is about the size of the two scans together.
If your backs are already in order
Some scanning apps and copiers reverse the order for you, and some people re-sort the stack before scanning the second side. Check the backs PDF: if its first page is the back of sheet 1, switch Backs are in reverse order off before interleaving.
Fixing common problems
Some back pages are upside down
Depending on how you turn the stack — over the long edge or the short edge — the backs can come out upside down. If every back is upside down, rotate the backs file 180° with Rotate PDF before interleaving. If only some pages are affected, interleave first and then turn the individual pages in Organize PDF.
One-sided sheets in the stack
If a sheet has nothing on the back, the simplest fix is to scan its blank back anyway, so the pairs stay matched. If you've already skipped it, the backs file is one page short and every page after that point pairs up with the wrong front. Insert a blank page at the right spot with Add Blank Pages, then interleave.
The two files have different page counts
Interleave alternates pages while both files still have pages left, then adds the rest of the longer file at the end. That's correct when the last sheets are single-sided, but if the mismatch is in the middle, fix it first as described above, or reorder the result in Organize PDF.
Pages are out of order in other ways
If the feeder grabbed two sheets at once or a page jammed, open the combined file in Organize PDF, where you can drag pages into place, delete duplicates and rotate any page.
Scanning with a phone instead
Phone scanning apps usually save each side as you go, so the order is already right and you only need one PDF. If your app saves photos instead, turn them into a PDF with JPG to PDF, where you can drag the images into order before converting.
Before you send it
- Flick through the result once — a missed page is much easier to fix now than after it's been submitted.
- Scans are large. If the document is going by email or into an upload form, see how to make a PDF smaller.
- Remove blank backs you don't need with Remove PDF Pages, especially if the recipient will print the document.
Questions
What does interleaving a PDF mean?
It means combining two PDFs by alternating their pages — page 1 of the first file, page 1 of the second, page 2 of the first, and so on — instead of putting one file after the other.
Can I interleave more than two PDFs?
Interleave PDF takes exactly two files: the fronts and the backs. To combine more documents one after another, use Merge PDF.
Do I need a duplex scanner?
No. A duplex scanner scans both sides in one pass, but scanning each side separately and interleaving the files gives the same result.