Two different kinds of locked PDF
PDFs support two separate passwords, and they are not the same problem.
An owner password leaves the document readable but sets permission flags that ask readers to block printing, copying or editing. The content itself is not meaningfully protected — the flags are a request that viewers choose to honour. This tool clears those.
A user password encrypts the document. It will not open at all without the password, and the content genuinely cannot be read without it. No browser tool can remove that, including this one. Any site claiming otherwise is either uploading your file to a server that attempts to crack it, or quietly failing.
Is this legal?
Removing restrictions from a document you own or have the right to use — a report you were sent, a form you need to print — is ordinary. Circumventing protection on material you have no right to access is not. Only use this on files you are entitled to.
What happens to the file
It is read into memory by your browser, rewritten without the permission flags, and handed straight back to you. No copy is transmitted, which you can confirm in your browser's network tab.