Password protecting a PDF doesn't require paid software. Both Mac and browser-based tools handle it for free, and the resulting protection is the same AES-256 encryption used by commercial tools.
On Mac: Preview Does It Built-In
Open the PDF in Preview, go to File โ Export as PDF, and click Show Details. You'll see options to set an owner password and a user password. The user password controls who can open the file. The owner password controls permissions โ whether the recipient can print, copy, or edit. Set one or both depending on what you need, then save.
Worth knowing: Preview uses older 128-bit RC4 encryption by default, not AES-256. For most purposes โ protecting a document from casual access โ this is fine. For sensitive financial or legal documents where strong encryption matters, a browser-based tool that specifies AES-256 is the better choice.
Browser-Based Tool: Works on Any Device, Stronger Encryption
WukongPDF's PDF Security tool runs in any browser โ upload the PDF, set a password, download the protected file. The encryption used is AES-256, which is the current standard. No account required, works on Windows, Mac, Chromebook, or mobile.
This is also the better option for Windows users, since Windows doesn't have a Preview equivalent built in. Microsoft Word can add a password to Word documents before saving as PDF, but it can't protect a PDF that already exists.
A Few Things Worth Knowing About PDF Passwords
Never send the password in the same email as the protected file โ that defeats the purpose entirely. Send the file first, then share the password through a different channel: a text message, a phone call, or a separate email. It's a small extra step that makes the protection actually meaningful.
PDF password protection controls access to the file, not what recipients do with the content once they have it. A person who opens the document can still take screenshots or photograph the screen. For documents where redistribution is a serious concern, password protection is one layer of a broader approach, not a complete solution by itself.
If you forget the password on your own document, recovery depends on how strong the password was. Older PDFs with 40-bit encryption can often be recovered with dedicated tools. Modern AES-256 protected files are effectively unrecoverable without the password. Write it down somewhere safe.