Guide
How to password protect a PDF
Open Protect PDF, add your file, type a password twice and press Protect PDF. The copy you download asks for that password every time it is opened, in any PDF reader. It uses AES-256 and runs in your browser, so neither the file nor the password is uploaded.
Open Protect PDFStep by step
- Open Protect PDF. Go to the Protect PDF page on PlumPDF. No sign-up and nothing to install.
- Add your PDFs. Drop one or more PDFs on the box. Up to 50 files of 100 MB each.
- Choose a password. Type it twice. Use at least 8 characters; a few unrelated words work well.
- Protect and download. Press Protect PDF and save the locked copy. Keep the password somewhere safe.
What does the password do?
A password on a PDF encrypts its content, so without the password the file is unreadable data. PlumPDF uses AES with a 256-bit key, the strongest encryption the PDF format offers, and the result opens in all major PDF readers. Your original file is not changed.
The password is an open password: it is needed to open the file. Once someone has it, they can read, print, copy and edit. No printing or copying restrictions are set, because those are honoured by some readers and ignored by others, so they are not real protection.
How do I choose a good password?
In practice the password is the weak point, not the encryption. Some habits help:
- Make it long. Four or five unrelated words are stronger and easier to remember than a short mix of symbols.
- Do not use a birthday, a name or a word from the file.
- Send the password by a different route from the file, such as a phone call or a message.
- Store it in a password manager.
What if I forget the password?
The file cannot be opened, by you or by PlumPDF. There is no recovery and no back door, which is the whole point of encryption. Keep the unprotected original somewhere safe, or keep the password in a password manager.
Why does the browser matter here?
It would be odd to send a file to a stranger in order to lock it away from strangers. Here the encryption is done on your device. The password is held in the page's memory only while the tool runs.
Protect PDF works on files that have no password yet. Files that already have one are skipped and listed. To change a password, remove the old one first with Unlock PDF.
Questions
Can I protect many PDFs with one password?
Yes. Add up to 50 PDFs and type the password once. The same password is set on every file, and you can download them one by one or as a zip.
Will the PDF look the same after protecting it?
Yes. Text, images, links and bookmarks are exactly as before, and the file size grows only slightly.
Does a password stop someone changing the file?
Only if they do not have the password. With it, they can edit the contents. For a document that must show it has not been altered, a certificate-based digital signature is the right tool.
Should I also remove hidden data before locking it?
It is a good idea. Use PDF Privacy Check first to clear the author name and other hidden data, then protect the clean copy.
Tools in this guide
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