How the protection works
The file is encrypted with AES-256, the strongest standard encryption in the PDF format, supported by Adobe Acrobat, Chrome, Edge, Firefox, Preview on Mac and phone PDF apps. Without the password the contents can’t be read. With it, the file opens normally.
The permissions under “Once opened, people can:” are different. They ask the reader app not to allow printing, copying or editing, and mainstream apps follow them, but they aren’t encryption: some tools ignore them. Treat them as a polite request; the password is the real lock.
Choosing a password
Length matters most. Current guidance from the US standards body NIST recommends at least 15 characters for a password used on its own. A few random words, such as river-candle-orbit-plum, are long and easy to type. The strength line under the box rates your password as you type. Avoid names, birthdays and phone numbers, which are the first things people try.
Send the password a different way from the file: if you email the PDF, send the password by text message or tell it in person. Keep a copy somewhere safe, because a forgotten password can’t be recovered, by us or anyone else. The file is only as safe as the password.
What isn’t changed
Once a file is locked, its reader app asks for the password every time it opens, on any device. Pages, text, images, links and form fields stay as they were; only the encryption is added, and the file name gets -protected at the end. If the PDF was already password-protected, unlock it first with Remove PDF Password and then set the new password.
Your file is opened and processed in your browser, so it is never uploaded to a server. That matters for a tool like this: the unprotected document and the password you choose stay on your device.