Scan double-sided documents with a single-sided scanner

Many home and office scanners with a document feeder only scan one side of each sheet. You don't need to feed pages one at a time to scan a double-sided document: scan the stack twice and let a tool put the pages back in order.

Step 1: scan the fronts

Put the stack in the feeder as usual and scan it to a PDF. You now have pages 1, 3, 5 and so on, in order.

Step 2: scan the backs

Take the stack out of the output tray, turn it over without reshuffling it, and scan again to a second PDF. Because the stack is upside down, the backs usually come out in reverse: the last page's back first.

Step 3: combine them

1. Open Alternate-Mix PDF.
2. Choose the fronts as the first PDF and the backs as the second.
3. Tick Reverse the second PDF if the backs came out in reverse order (they usually do if you flipped the stack).
4. Click Mix pages and download the result.

If some backs are upside down, fix them with Rotate PDF. If a sheet had a blank back, remove the empty pages with Delete PDF Pages.

If the counts don't match

If one scan has more pages than the other (a sheet stuck, or a double feed), the tool still mixes what it can and adds the leftover pages at the end, and it tells you. Check the result and rescan the sheet that went wrong.