Combining PDFs is the most mechanical job in this category and the one most often made annoying. The usual pattern is an upload queue, a progress bar, a wait, and a download — for an operation that is essentially copying pages from one list into another.
Here it happens in the browser, immediately, with both documents visible. Pages that came from the file you added are tagged, which sounds like a small thing until you are looking at a forty-page combination and trying to work out where the appendix started.
Three arrangements cover the real cases. At the end is the default and the common one — a covering letter followed by the thing it covers, or a report followed by its appendix. At the start is the same idea inverted. Alternating exists because of a specific, extremely common frustration: a document scanned one side at a time, leaving you with all the fronts in one file and all the backs in another. Interleaving them by hand is sixty drag operations. Here it is one choice, with a warning that it assumes the second file is in the same order — because if the backs were scanned in reverse, that is worth catching before rather than after.
The order is the one thing to check, and it is stated rather than assumed: the files combine in the order you opened them, named on the confirm before anything happens. And because a signed document’s signature does not survive any recombination, when that applies it is said on the same line — a fact, never a warning.
Nothing is overwritten. Both files you started with are untouched on your disk, and what you download is new. If the page order is not quite right, the page list lets you drag any page anywhere afterwards, and rotating a page that came in sideways is one click.
No upload, no count, no watermark, and no account.