If you have a PDF that's password-protected and you know the password, you can remove it to make the file easier to open, share, or edit going forward. Since PDF passwords protect sensitive documents, using a tool that processes the file on your own device, rather than uploading it to an unknown server, matters for privacy.
No, it removes a password you already have. It doesn't attempt to guess or crack unknown passwords. That would require different techniques and isn't something this tool does.
Yes. The PDF and its password are processed entirely in your browser using WebAssembly. Nothing is uploaded to a server at any point.
Yes, since it runs in your browser rather than as installed software, it works the same way on Mac, Windows, Linux, and mobile browsers.