Combining PDFs is a two-minute job that most people do with the first website they find. That is fine for a holiday itinerary and genuinely risky for an employment contract, a payslip or a medical report.
The question is not whether a service is trustworthy. It is whether the file needs to leave your computer at all — and for merging, it does not.
A classic online merger sends your documents to a server, joins them there and sends one back. That means your file sits on someone else's disk, passes through their logs and backups, and is governed by a retention policy you have to take on faith. Reputable services delete files after a few hours; you cannot verify it.
For most documents nobody cares. For anything with a salary, an address, a signature or a diagnosis in it, you have handed a copy to a third party with no way to take it back.
Joining PDFs does not need a server. The work is copying page objects from several documents into one, which any modern browser does instantly. Abzar's merge tool works this way: you add the files, the pages are copied in the order you added them, and the combined document is assembled locally. Nothing is uploaded, so there is nothing to delete afterwards.
Because pages are copied rather than re-encoded, nothing degrades either — text stays selectable and scans keep their original resolution.
Files are joined in the order you add them, so add them in the order you want them read. If you need only part of a document, split it first and merge the piece you want.
Password-protected PDFs cannot be read for merging while they are encrypted. Remove the password first, merge, then protect the result again if it needs to stay locked.
Two signs are reliable. If the tool shows an upload progress bar for a large file and then a waiting spinner, the file went to a server. If the result appears almost instantly for a file far too big to have been uploaded on your connection, the work happened locally.
The other check is the privacy policy: a local tool has nothing to say about how long it keeps your files, because it never receives them.