Password-protect a PDF without uploading it
The documents people most want to encrypt are the ones they least want to hand to a stranger: contracts, medical letters, bank statements, anything with an identity number in it. There is an obvious problem with uploading such a file to a website in order to secure it — for the whole time it is in transit and on that server, it is not secure at all, and you have no way to verify what happened to it.
This tool avoids the question entirely. The encryption engine is qpdf, a long-standing open-source PDF library, compiled to WebAssembly and running inside your browser. Your file is read into memory on your own device, encrypted there, and written back out. Nothing is transmitted.
What the password actually protects
PDF defines two different things that both get called “protection”, and the difference matters.
The open password is real cryptography. The document's content is encrypted with AES-256 and the key is derived from your password. Without it there is nothing to display — a reader cannot show you the document because it cannot decrypt it.
Permissions — the print and copy restrictions — are not. They are recorded in the file as flags, and a reader is expected to honour them. Most do. Some do not, and nothing in the format compels them to. Any tool implying that “disable printing” is enforced is overstating what PDF can do. Treat those settings as a clear statement of intent to a cooperative reader, and the open password as the actual control.
Choosing a password
AES-256 is not the weak point; your password is. A four-character password on a 256-bit cipher is a four-character password. Length does more work than complexity — a passphrase of several unrelated words beats a short string of substituted characters, and is easier to type on a phone.
Whatever you choose, record it somewhere before you close this tab. There is no recovery path, by design. Secure Password Generator will produce one and it, too, runs entirely on your device.
Related tools
To take a password off a document you can already open, use Unlock PDF. To mark a document visually rather than encrypt it, use Watermark PDF. To sign it, use Sign PDF.