Some PDFs open fine but refuse to print or let you copy a line. Others ask for a password you already have and want typed every single time. This takes either kind of lock off and hands back an ordinary file. It runs on your own machine, which matters more here than anywhere else on this site: a file worth locking is a file worth not uploading.
The security engine (about 1.3 MB) loads only when you drop a file in, and it runs here rather than on a server — which matters more than usual when the file has a password on it. Your file is read on your own machine and never sent anywhere — open your browser's network tab and watch while you use it.
No, and it is not trying to. There is no guessing, no dictionary and no brute force here, and none will be added. It removes restrictions from a file that already opens on your machine, or decrypts one whose password you type in yourself.
A PDF can carry an open password, which is asked for before you see anything, and an owner password, which leaves the file readable but blocks printing or copying. The second kind is the one most people run into. Both come off here; only the first needs you to know the password.
No. The engine is a 1.3 MB WebAssembly build of qpdf that downloads once and runs inside this page. Open your network tab while you use it: the engine comes down, your file does not go up. Nothing you type as a password leaves the page either.
No. Pages, text and images are written back as they were; only the encryption layer is dropped. The file size usually shrinks a little, because the encryption overhead goes with it.
These all work the same way — in your browser. Most are free; the ones marked as needing a plan are not.