How to password protect a PDF on Mac — Preview, or a browser tab
To password protect a PDF on a Mac, open it in Preview, choose File, then Export, click Permissions and select Require Password To Open Document. Or open Protect PDF in Safari, type a password twice and download an AES-256 copy, with nothing uploaded.
A Mac can put a password on a PDF without any extra software, and Preview is the first answer for most people. What the usual guides leave out is how strong the result is, which of the Mac's several locks actually keeps a stranger out, and why a password does not make a file non-editable. This covers both routes and those three questions.
How do I password protect a PDF on Mac?
In Preview, open the PDF, choose File, then Export, and click Permissions. Select Require Password To Open Document, type the password, retype it to verify, then save the exported copy. That is Apple's own route as Apple Support describes it. We have not walked through it on a Mac for this guide.
The same Permissions panel also sets an owner password, and Apple says you select the checkbox next to an action, such as printing or making changes, to allow it without that owner password. Older guides show an Encrypt checkbox in the Export dialog instead of a Permissions button; it does the same job on older versions of macOS.
How can I password protect a PDF document for free?
Open Protect PDF in Safari or Chrome, choose the file, type a password in both boxes and click Protect PDF. An encrypted copy named yourfile_protected.pdf downloads and the original is left as it was. It is free, with no account and no app, and it runs inside the browser tab.
Open the tool
Go to Protect PDF in Safari, Chrome or any other browser on the Mac. There is nothing to install.
Choose the PDF
Drag it from Finder onto the drop area, or click to pick it. The two password boxes appear once the file has loaded.
Type the password twice
Enter it in Password and again in Confirm password. The button stays disabled until both boxes are filled, and "Passwords don't match yet." appears while they differ.
Protect and save
Click Protect PDF. The encrypted copy downloads with _protected on the end of its name. Your original file is not changed.
Does locking a file in Finder password protect it?
No. The Locked checkbox in Finder's Get Info window, and the Lock option Apple documents for Preview, stop that copy being changed or deleted by accident. Neither asks for a password to open the file. A locked PDF still opens for anyone who can reach it, and the copy you email or upload opens for whoever receives it.
Only a password set inside the PDF itself, from Preview's Permissions panel or a tool like Protect PDF, encrypts the contents so that they cannot be read without it. That is the one to use before a document leaves your Mac.
How strong is the password protection on a Mac PDF?
Protect PDF encrypts every file with AES-256, the strongest cipher the PDF format defines, whatever version the file declares. That matters on a Mac because PDFs saved by Preview commonly declare the old version 1.3, and the library underneath, left to itself, gives version 1.3 files weak 40-bit encryption. The tool overrides that and forces AES-256.
We checked this with the real code. We stamped a 40-page test file with a version 1.3 header and a title, protected it, and read it back with a different PDF library from the one that wrote it. It came out AES-256, the title was nowhere in the raw bytes, the wrong password was refused, and with the right one the title and the text on page 40 came back intact. The file went from 66,201 bytes to 66,209.
The cipher is not the weak point; the password is. We pulled the password check value out of a protected file and recomputed it by hand: one SHA-256 hash of the password plus an 8-byte salt matched it exactly. That is revision 5 of the PDF password scheme, where testing a guess costs a single hash. The later revision 6, from PDF 2.0, repeats the hashing to slow guessing down. The tool also sets no minimum length and will accept one letter. Use a long passphrase.
How do I make a PDF non-editable on a Mac?
A password does not really do it. Preview's owner password and its permission checkboxes ask the viewer to block printing or changes, and a viewer can choose to ignore that request. The open password is the only part that is enforced by encryption, which what a PDF password actually protects explains.
Protect PDF does not offer the edit lock at all. We read the permission record in a protected file: printing is marked allowed, while copying, editing, commenting and form filling are marked off. But the password that opens the file is also its owner password, so anyone who can read it holds the key that lifts those marks. For a copy that is hard to edit, flatten the pages to images instead.
Can I change the password on a PDF that already has one?
Not in one step. Protect PDF turns an already protected file away with "This PDF is password-protected. Unlock it first, then try again." Remove the old password with Unlock PDF, as how to remove a password from a PDF on Mac walks through, then protect the unlocked copy with the new password.
A forgotten open password cannot be recovered, by you or by us. The tool stores it nowhere, and the contents are encrypted with it. Keep it in a password manager, or in the Passwords app on the Mac, before you send the file.
Is it safe to password protect a PDF in a browser?
Here, yes. The document and the password stay in the browser tab: Protect PDF encrypts the file on the Mac, and the site has no server to receive either. In our test, a fillable field kept its value through protection too, because the tool encrypts the file as it is rather than rebuilding it.
Adobe's free online protect tool is the other common answer, and it sends the file to Adobe to be encrypted. Is it safe to upload a PDF online covers that trade-off. For the paid Acrobat route, see how to password protect a PDF in Adobe Acrobat; the short general guide is how to password protect a PDF.
