Alternate Merge
Interleave the pages of two PDFs, such as front and back scans, into one file in your browser, with an option to reverse the second file. No upload.
Upload PDF Files
About This Tool
Alternate Merge builds one PDF by taking pages in turn from each file: page 1 of the first file, page 1 of the second, then page 2 of each, and so on. It is mainly used to join a scan of front sides with a separate scan of back sides. Bookmarks and properties of the originals are not carried over.
The order of the files in the list decides the order of the pages. Drag a row, or use its arrow buttons, so the file that should open the document is number 1. You can add up to 10 files; with three or more, one page is taken from each file per round. When a file runs out of pages, the remaining pages of the longer files follow on, so a sheet that was skipped or fed twice in one scan puts every later pair out of step.
Reverse second document reads the file in position two from its last page to its first. Turn it on when the backs were scanned by turning the whole stack over, so the back of the final sheet came out first. It never changes the first file or any file after the second. If the backs come out upside down, turn them afterwards with Rotate PDF; for other order problems, Reverse Pages and Organize PDF can help before or after.
The files are combined in your browser tab and are not uploaded. The download, interleaved.pdf, is a new document, so compare its page count with the sources and keep the original scans until the combined file has been checked. To join whole files one after another instead, use Merge PDF, covered in How to Merge PDF Files Into One.
How to Use
Add the scans
Drop the fronts file and the backs file on the upload area, or click it to browse. Up to 10 PDFs can be added.
Check the order
Files are numbered in the order their pages will be taken. Drag a row or use the up and down arrows so the right file is number 1. The cross removes a file.
Reverse if needed
Once two files are listed, Interleave Options appears. Tick Reverse second document if the second file runs from the last sheet to the first.
Interleave and download
Click Interleave PDFs, save interleaved.pdf with the download button, and page through it to confirm that fronts and backs alternate correctly.
Use Cases
Double-sided paper records
Scan a stack of two-sided forms on a single-sided scanner in two passes, then interleave the passes into one file in reading order.
Backs scanned in reverse
When the stack was flipped for the second pass, reverse that file during the merge instead of reordering pages by hand.
Source and translation side by side
Put each page of an original next to the matching page of its translation so a reviewer can compare them in sequence.
Frequently Asked Questions
Which file should be first in the list?
The file whose first page should open the document, which for duplex scans is normally the fronts. Pages are then taken in turn: page 1 of file 1, page 1 of file 2, page 2 of file 1 and so on.
What does Reverse second document do?
It reads the second file in the list from its last page to its first before interleaving. Use it when the backs were scanned by turning the stack over. It applies only to the file in position two.
What if the two files have different page counts?
Pages alternate until the shorter file runs out, then the rest of the longer file is added in order at the end. A skipped or double-fed sheet in one scan pairs every later page wrongly, so compare the page counts before you merge.
Can I interleave more than two files?
Yes, up to 10. One page is taken from each file in list order on every round. Only the second file can be reversed.
Are bookmarks and document properties kept?
No. The pages are copied into a new document, so the bookmarks, title, author and other properties of the sources are not carried over. Page content is copied without re-compression.
Why does it stop with an encryption message?
One of the files is password-protected or carries security settings, and the message names it. Remove the protection with Decrypt PDF, which needs the password, then add the unlocked copy.