How to Merge Front and Back Scans Into One Double-Sided PDF

Many home and office scanners can only scan one side at a time. The usual trick is to scan all the fronts, flip the stack, and scan all the backs. That leaves you with two PDFs (or one PDF with all the fronts followed by all the backs) instead of one document in reading order. Here's how to combine front and back scans into a proper double-sided PDF in one step.

Merge front and back scans in one step

1. Open Merge PDF.
2. Add the fronts file first and the backs file second. Check the list order; use the arrows to swap them if needed.
3. Tick Front and back scans: alternate pages.
4. Leave The backs are in reverse order ticked if you flipped the whole stack to scan the backs, which is the usual case (see below). Untick it if your backs are already in the same order as the fronts.
5. Click Merge. The result alternates pages: front 1, back 1, front 2, back 2, and so on.

It all runs in your browser. The scans aren't uploaded, which matters for contracts, medical letters and ID documents.

Why the backs come out in reverse order

Say you scan a 3-sheet stack, fronts first: the scanner reads front 1, front 2, front 3. Then you flip the whole stack over and feed it again. Now the sheet that was on the bottom is on top, so the scanner reads back 3, back 2, back 1.

That's why reverse order is the default. A quick test if you're not sure: open the backs file. If its first page is the back of your last sheet, they're reversed.

Some scanners and apps scan the backs in the same order as the fronts, for example if you re-stack the pages instead of flipping them. In that case, untick the reverse option.

If fronts and backs are in one file

If your scanner saved everything into a single PDF (all fronts, then all backs), split it into two files first:

1. Open Split PDF and choose Extract pages.
2. Extract the fronts: for 12 sheets, pages 1-12.
3. Extract the backs: pages 13-24.
4. Merge the two files as described above.

Check the result

Flip through the first few pages and the last few:

• Front and back don't match? The files are probably in the wrong order in the list (backs first). Swap them and merge again.
• Backs in the wrong order? Toggle The backs are in reverse order and merge again.
• Blank pages where some sheets had nothing on the back? That's normal for duplex scans of mixed documents. Remove them in Organize Pages, which marks blank pages automatically.
• Some backs upside down? If you flipped the stack the other way, backs can come out rotated 180°. Fix them with Rotate PDF.
• Different number of fronts and backs? If a sheet got skipped or double-fed, the pages fall out of step from that point. Any extra pages from the longer file are added at the end. Re-scan the missing sheet, or fix the order in Organize Pages.

Scanning tips that save time

• Number your sheets lightly in pencil before scanning a long stack. A mismatch is then obvious.
• Scan fronts and backs straight after each other, without adding or removing sheets in between.
• Check whether your scanner has a duplex or "manual duplex" mode. Some scanner software interleaves for you. If yours doesn't, the merge above does the same job.

FAQ

How do I merge front and back scans into one PDF? In Merge PDF, add the fronts file then the backs file, tick Front and back scans, and merge. Pages are alternated front, back, front, back.

Why are my back pages in reverse order? Flipping the stack to scan the backs puts the last sheet on top, so the backs are scanned last-to-first. The reverse option, on by default, corrects this.

Can I make a double-sided PDF from single-sided scans for free? Yes. Merge PDF is free, with no watermark and no sign-up.

What if I have more fronts than backs? The pages alternate as far as both files go, then the remaining pages of the longer file are added at the end. Check the result and fix any gaps in Organize Pages.

Will merging reduce scan quality? No. Pages are copied as they are; nothing is compressed.