How to merge PDF files without uploading them
· 5 minute read
Joining PDFs rarely needs a server. Here is how to merge them in the right order, how to check the seams, and how to tell whether a site is sending your files away.
Search for "merge PDF" and you will find hundreds of sites that will do it for you. Nearly all of them ask you to upload your files first. For a restaurant menu that hardly matters. For a signed contract, a set of payslips or a passport scan, it matters a great deal, and the strange thing is that it is not necessary: joining PDFs is a job a browser can do on its own.
Why merging does not need a server
A PDF is a container of pages. Joining two of them means copying the pages from one container into another and writing a new index. There is no heavy computation in that, no recognition, no layout. Modern browsers can read and write PDFs directly, so a page can take your files from your disk, build the merged file in memory and hand it back to you as a download. Nothing has to leave your machine.
Sites that upload do so for business reasons rather than technical ones. A server lets them count your usage, require an account, put a limit on the free tier and, in some cases, keep a copy. None of that helps you.
How to merge them in the right order
The order is the whole job. Before you start, decide the sequence on paper: cover letter, agreement, schedules in numerical order, signature page. Then add the files, check the list from top to bottom and move anything out of place before you press the button.
- Name your files so that they sort sensibly, such as 01-cover, 02-agreement, 03-schedule-a. If you select them all at once they will arrive in that order.
- Add all the files in one go if you can. Adding them one by one makes it easier to put one in the wrong place.
- Check the list for duplicates. A file that was added twice becomes a document with a repeated section.
- Merge, then look at the result before you save it.
Check the seams
The places where one file ends and the next begins are where mistakes live. A page from the wrong file, a missing page, or a page that is the wrong way up all show up at the seams. A good merge tool shows you the finished document with each page labelled by the file it came from, so you can look at only the boundaries instead of reading the whole thing.
Pay particular attention to page orientation. A landscape page from one file sitting between portrait pages from another is normal, but a page that is upside down means the source file was scanned that way, and it is better to fix it before the document goes out.
What to do about the size afterwards
Merging does not make anything smaller. The result is about as big as the sum of its parts, which can be a problem if you are going to email it. If the file is too large, run it through Compress PDF, which re-encodes the pictures and leaves the text alone. We wrote about how that works in how to reduce a PDF’s file size.
Number the bundle once, at the end
If the finished document needs page numbers, add them after the merge, not before. Numbering each file separately gives you a restart at every seam, and then "see page 3" no longer points to anywhere in particular. Merge first, then use Page numbers once for the whole thing.
How to tell whether a site uploads your files
There is a one-minute test. Load the page, then turn off your internet connection, then try the merge. If it still works, nothing was uploaded, because there was nowhere to send it. If it hangs or shows an error about the network, your file was on its way to someone else’s computer. We explain more in is it safe to upload PDFs online.
Tools mentioned
- Merge PDF — Several files, one document, in the order you put them in.
- Compress PDF — A smaller file that still reads like the original.
- Page numbers — Numbering across the document, in the corner you pick.