How to password protect a PDF

Free ways to add a password

How to password protect a PDF with Protect PDF

1. Open Protect PDF and choose your PDF. The file opens in your browser and isn't uploaded.
2. Type a password, then type it again. It needs at least 6 characters, but longer is much safer (see below). Typing it twice catches typing mistakes.
3. Press Protect PDF, then Download protected PDF. The new copy is encrypted with AES-256. Your original stays as it was.
4. Test it. Open the downloaded file. It should ask for the password before showing anything.

If your PDF already has a password, Protect PDF tells you so. To change the password, remove the old one with Unlock PDF (you'll need to know it), then protect the unlocked copy with the new one.

Want to make the file smaller too? Compress PDF first, then protect it. A protected file is harder for other tools to change.

Other free ways

Microsoft Word on Windows

If the document started in Word, you can add the password while saving it as a PDF. Go to File > Save a Copy (or Save As), choose PDF, then More options > Options, and tick Encrypt the document with a password. Word then asks you to type the password twice. Microsoft's guide for Word on a Mac doesn't show this option, so on a Mac use Preview or Protect PDF instead.

Preview on a Mac

Open the PDF in Preview and choose File > Export (give it a new name if you want to keep an unlocked original). Click Permissions, tick Require Password To Open Document and type the password twice. In the same window you can also stop people printing or copying without a second, "owner", password. Click Apply, then Save.

Adobe Acrobat

The free Adobe Acrobat Reader can't add a password. Adobe's Reader help says security settings "cannot be set in Reader". The paid Adobe Acrobat can, from Protect Using Password.

Open password or permissions password?

PDFs can have two kinds of password, and they do different jobs:

• Open password (also called a user password): nobody can see the document without it. This is what you want for private documents, and it's what Protect PDF adds.
• Permissions password (also called an owner password): the file opens freely, but printing, copying or editing is limited unless you know it. It doesn't keep the contents private, because anyone can read the file.

Adobe explains that if a PDF has both, either password opens it, but only the permissions password can change the limits.

How strong is PDF encryption?

Modern PDF encryption uses AES with a 256-bit key, which Adobe has used since Acrobat X. Protect PDF uses AES-256 too. Older settings (128-bit RC4, from Acrobat 6) are much weaker, so avoid them if you get a choice. In practice, the weak point is almost always the password, not the encryption: a short or common password can be guessed, however strong the lock.

Choosing a good password

• Use three random words. The UK's National Cyber Security Centre recommends this: three unrelated words together are long, hard to guess and easy to remember.
• Avoid personal details such as birthdays, family or pet names, and common passwords. The NCSC notes that swapping letters for symbols (0 for o) adds little.
• Don't reuse the password from your email or bank.
• Send it separately. If the password is in the same email as the file, anyone who gets the email gets both.

If you forget the password

Write it down somewhere safe or keep it in a password manager. Adobe is clear that a forgotten password can't be recovered from the PDF, and nor can we: Protect PDF never sees your password. The simplest protection is to keep your original, unlocked file somewhere safe, so you can lock a new copy if you need to.