You're about to email a bank statement, a signed contract, or a scan of your ID — and you'd rather it not sit readable in someone's inbox forever if the message goes to the wrong person. A password on the PDF fixes that: without it, the file won't even open. The catch most guides miss is that a document worth locking is exactly the kind you shouldn't hand to a random "PDF password" website to do it. You don't have to — your own computer can encrypt it locally.

Mac encrypts PDFs out of the box (Preview → Export), and on Windows the free LibreOffice does it offline. Both lock the file on your machine, so the sensitive document never leaves it. Here's the how, plus what a PDF password does and doesn't protect.

Two kinds of password, and only one of them locks anything

A PDF open password encrypts the file: no password, no reading it — solid protection in transit and at rest. What it is not is a substitute for the recipient handling it responsibly once they've opened it. And "permissions" passwords (which try to block printing or copying while still letting the file open) are weak and easily stripped — rely on the open password, not permissions, for anything that matters.

That is also why the common way of "protecting" a PDF is backwards: dragging it into an online locker uploads an unprotected copy to someone else's server first. If the file is worth encrypting, encrypt it on your own device. The three ways below all do.

Mac (built in, no download)

macOS Preview can encrypt any PDF:

  1. Open the PDF in Preview.
  2. File → Export.
  3. Tick Encrypt, type a password (twice), and Save.

From now on the file asks for that password before it opens. That's it — nothing installed, nothing uploaded.

Windows (free, offline — LibreOffice)

Windows has no built-in PDF encryptor, but the free, open-source LibreOffice does it locally:

  1. Open the PDF (or the source document) in LibreOffice.
  2. File → Export As → Export as PDF.
  3. On the Security tab, click Set Passwords, enter an open password, and export.

LibreOffice is a one-time free download that runs entirely offline — a safe, permanent tool to keep for this. (If you have Adobe Acrobat Pro, File → Protect works too, but you don't need paid software.)

On your phone

  • iPhone: open the PDF in the Files app, tap the share/more menu, and choose Lock PDF — set a password. iOS encrypts it in place.
  • Android: most PDF reader apps (and some file managers) have an Encrypt / Set password option in the share or more menu; pick a strong password and save a new copy.

A password that helps

  • Strong and unique. The file is only as safe as the password; a short, reused one defeats the point.
  • Sent separately. Never in the same email as the file. Text it, say it on a call, or use another channel.
  • Backed up. Keep an unlocked master copy somewhere safe. A properly encrypted PDF without its password is unrecoverable, which is the feature working as designed.