Password protect PDF
Lock a PDF with a password using AES-256. Drop as many PDFs as you like.
Runs on your device. The file is never uploaded.
Password protect PDF encrypts a document with AES-256 through qpdf, so the file cannot be opened without the password you choose. The owner password governs permissions and copies your open password when you leave it blank. No permission flags are passed, so a reader who can open the file can still print and copy from it.
Questions
What encryption does it use?
AES-256, applied by qpdf. That is the strongest encryption the PDF specification defines, and it is the same thing Acrobat produces when you ask for 256-bit security.
What is the difference between the two passwords?
The first one is needed to open the document at all. The owner password governs permissions, printing, copying, editing, for readers that honour them. Leave it blank and it is set to the same value as the open password.
Can I use a blank password?
No, and the tool refuses rather than pretending: "choose a password to lock the PDF with". A PDF encrypted with an empty user password opens for anybody, so it would be claiming a protection it does not have.
Does my password get sent anywhere?
No. The password is typed into a field on this page, passed to qpdf compiled to WebAssembly running in this tab, and never leaves it. It is also passed to qpdf as a keyword option rather than a positional one specifically so that a password beginning with "--" can never be re-read as a flag and echoed back in an error message.
Can I protect a PDF that is already encrypted?
No, "already password-protected, unlock it first". Unlock it with the password you have, then run this.
If I forget the password, can you recover it?
No. There is no recovery, no reset and no copy of your file anywhere. Losing the password means losing the document, which is what encryption means.