PDF Tools·3 min read

Merging Front and Back Scans Into One PDF

Plenty of office scanners only read one side of the paper. The usual workaround is to scan the fronts, flip the stack and scan the backs, which leaves you with two files and a document that no longer reads in order. Dragging pages into place works for three sheets, but not for forty. The Alternate and Mix PDF tool takes pages from each file in turn, so fronts and backs interleave automatically, and it can reverse a file that came out last page first. This guide explains why the order breaks, how to set up the mix, a worked example and the checks that catch a skipped sheet.

Open the free Alternate and Mix PDF Pages All insights

Why one-sided scanning scrambles the order

The flip reverses the stack.

Scan five double-sided sheets on a simplex feeder and the first pass gives front 1 to front 5. To reach the backs you turn the whole stack over, which puts sheet 5 on top. The second pass therefore gives back 5, back 4 and so on down to back 1.

Neither file is wrong on its own. The fronts are in order and the backs are in perfect reverse. What you need is a third file that takes one page from each in turn and reads the backs from the end, which is exactly what alternating does. Some duplex-capable scanners have a mode that does this for you, but many shared office machines do not, and phone scanning apps almost never do.

How alternating works

Take, switch, repeat.

Add two or more PDFs and each gets a letter in list order, A, B, C. The tool takes pages from A, then from B, then from C, and back to A, until the files run out. The New order list shows the result as tokens like A1 B5 A2 B4 before anything is saved, so you can check the pattern at a glance.

Each file has three settings. Reverse reads its pages from last to first. Per turn sets how many pages are taken before switching. Pages limits the file to a range, such as 2-10. A final choice decides what happens when one file runs out: keep adding pages from the others, or stop at the end of the shortest file.

A worked mix: five sheets, two files

Ten pages in reading order.

The files are fronts.pdf with 5 pages and backs.pdf with 5 pages, scanned as described above. Drop both on the tool, check that fronts.pdf is row A, and click Double-sided scan preset. That ticks Reverse on row B and sets one page per turn.

The New order list reads A1 B5 A2 B4 A3 B3 A4 B2 A5 B1. Page 5 of the backs file is the back of sheet 1, so that sequence is front 1, back 1, front 2, back 2, through to back 5. Click Mix & download and mixed.pdf holds ten pages in reading order, with every page copied as it was scanned.

Beyond scans: other ways to mix pages

Per turn does the heavy lifting.

Alternating is useful whenever two documents should be read side by side. Put a translation after each page of the original, a marking sheet after each student answer, or a separator page after every chapter.

Per turn handles uneven patterns. With A at 2 and B at 1 you get two pages of A, then one of B, which suits two-page forms that each need a single cover note. With three files, such as English, French and German versions of a manual, one page from each in turn builds a trilingual copy for proofreading.

Checks that catch a bad mix

Count first, mix second.

A mixed file is only as good as the scans that went in. Before downloading:

Compare the page counts on the two rows. Different counts usually mean a sheet was skipped or two were fed together, and every pair after that point will be off by one.
Keep blank backs in the scan. Deleting them first breaks the pairing, so remove them afterwards with Delete PDF Pages and its blank page finder.
Look at the last tokens of the New order list. If they end with a run from one file, the counts did not match.
Open the result and check a page near the middle, not just the first two. Errors in the feeder often appear halfway through a stack.

Alternate and Mix next to Merge and Organize

Weave, join or rearrange.

Merge PDF joins whole files end to end, which is right for chapters or attachments. Organize PDF Pages lets you drag individual pages anywhere and has a Mix files sort for simple A1, B1 alternation. Alternate and Mix is the tool for the double-sided job because it adds reversing, several pages per turn and ranges per file. If some backs come out upside down, fix them afterwards with Rotate PDF using Even pages and Rotate 180. The files never leave your browser at any step.