How to combine double-sided scans into one PDF
Updated
Contents
A scanner without a two-sided feeder scans one side at a time: you feed the stack for the fronts, turn it over and feed it again for the backs, and end up with two PDFs. To put them back in reading order, open Merge PDF, add the fronts file and then the backs file, and set Page order to Alternate pages. The merged PDF takes one page from each file in turn.
When you turn the stack over, the last sheet is usually scanned first, so the backs file runs backwards. Tick Reverse page order on that file and its pages are turned round before they are paired.
It runs in your browser, so your scans stay on your device. Each PDF can be up to 100 MB.
Step by step
- 1Open the Merge PDF tool and drop the PDF of front sides and the PDF of back sides where it says Drop your PDF here, or click browse. They stay on your device.
- 2Check the list: the fronts file should be at the top. Drag a row, or use its arrow buttons, to move it.
- 3Under Page order, choose Alternate pages in place of One after another.
- 4If the backs were scanned last sheet first, tick Reverse page order on the backs file's row.
- 5Press Merge 2 PDFs, then Download merged PDF. The copy is named after the top file, with -merged added.
How the pages are paired
With Alternate pages, page 1 of the top file comes first, then page 1 of the next file, then page 2 of each, and so on. Fronts 1, 2 and 3 with backs 1, 2 and 3 come out as front 1, back 1, front 2, back 2, front 3, back 3.
If one file is longer, its extra pages go at the end in their own order. The same works for more than two files: one page from each in turn, top file first.
When to tick Reverse page order
Reverse page order appears on each file's row once Alternate pages is chosen, and turns that file's pages round before they are paired. To tell whether the backs need it, open the backs file and look at its first page: if it shows the back of the last sheet, tick it.
Each row's Pages box still works in this mode. The pages you type are taken in the file's own order first, and only then turned round.
Bookmarks, blank backs and fixing the order later
A bookmark for each file and a contents page are not offered with Alternate pages, because each file's pages are spread through the result. Bookmarks already in your files are kept and still open the right pages.
Sheets printed on one side only leave blank backs in the merged PDF. Keep them while merging, because every back has to stay next to its front for the pairs to line up, and take them out afterwards with Delete Pages. If a page still lands in the wrong place, Organize Pages lets you drag it to where it belongs.
Common questions
How do I merge front and back scans into one PDF?
Add the fronts file first and the backs file second, choose Alternate pages under Page order, and merge. The result takes front 1, back 1, front 2, back 2, and so on.
Why are the back pages in the wrong order?
The stack was turned over to scan the backs, so the last sheet went through first. Tick Reverse page order on the backs file's row and merge again.
What if the two files have different numbers of pages?
The extra pages of the longer file go at the end. A page missing from the middle of one file moves every pair after it, so check that both files hold one page per sheet, blank backs included.
Can I still add bookmarks or a contents page?
Not with Alternate pages: a file's pages are spread through the result, so there is no one page for its bookmark to open. With One after another, both options are offered.
Are my scans sent to a server?
No. The PDFs are read and merged in your browser tab and the copy is saved on your device; they are not sent to pdf.xyz.