What actually happens when you "merge PDFs online"
Most PDF merge tools follow the same pattern: you upload your files, a server stitches them together, and you download the result. For a signed contract, a tax document, or anything else you'd rather not hand to a third party, that upload step is the part worth questioning - not because merging PDFs is inherently risky, but because it's a task that doesn't require a server at all.
A PDF is just a structured file format. Combining the pages of two PDFs into one is something a JavaScript library can do directly in the browser, reading and writing the files locally, with no network request involved.
Merging PDFs in the browser instead
PDF Merger works this way: pick your files, drag them into the order you want, and it produces the combined PDF locally. The files never leave your device - there's simply no upload step in the code path to begin with.
A couple of things worth knowing before you merge:
- Order matters, and it's whatever you set it to. Most tools merge files in the order you added them, not alphabetically - double check the final order before downloading, especially with more than two or three files.
- Password-protected PDFs need to be unlocked first. If a file has a password on it, a merge tool generally can't read its contents until it's decrypted - remove the password (locally, same idea) before merging.
- Large scanned PDFs merge fine, but take a moment. A merge of image-heavy scanned documents is doing more actual work than a merge of a few text-based pages - it's normal for it to take a few seconds longer.
The pattern to look for
None of this is specific to PDF merging. The same reasoning applies to extracting pages, converting formats, compressing images, or almost any "take a file, transform it, give it back" tool. If the transformation itself doesn't need a server - and most simple ones don't - the tool shouldn't require one either.