How to password protect a PDF
Unlike removing a password, adding real encryption usually needs a dedicated tool rather than a built-in export option. Here is what works.
Why this is harder than removing a password
Removing a password from a PDF you can already open is often a save-or-print trick, because the reader has already decrypted the content and just needs to re-save it. Adding a password means encrypting the file from scratch with a real cipher, which most everyday apps, including Preview on a Mac and the Photos or Files apps on Windows, simply do not offer as a built-in feature.
On a Mac with Preview
Preview can add a password: open the PDF, choose File, Export, check Encrypt, and set a password. This uses standard PDF encryption and is the one common exception to "your OS can't do this natively" below, but it is Mac-only.
On Windows
There is no equivalent built into File Explorer, the Photos app, or Edge's PDF viewer. Microsoft Word can save a document as a password-protected PDF (File, Save As, PDF, then Options, Encrypt the document with a password), but only if the PDF started as a Word file; it cannot add a password to a PDF you did not create in Word.
On an iPhone or Android
Neither platform has a native way to add a password to an existing PDF. The Files app on iPhone and most Android file managers can only view and organize PDFs, not encrypt them.
A browser tool that actually encrypts the file
Stayput's protect PDF tool sets a real password with AES-256 encryption directly in the browser: the file is encrypted on your device before it is ever saved, so the unprotected version never leaves your computer or gets uploaded anywhere to be locked. It works in any modern browser, including on Windows and Android where there is no built-in option, and handles several PDFs in one batch.
Questions
Is browser-based PDF encryption actually secure?
Yes, when the tool uses a standard, real cipher like AES-256 rather than just hiding the content. The key difference versus an online converter is whether the file is uploaded to add the password; if it is not, the plain file never leaves your device.
Can I password protect a PDF for free?
Yes, both Preview's built-in export on Mac and a browser-based tool that runs locally are free, unlike some online converters that charge or limit file size.
What is the difference between an open password and a permissions password?
An open password is required just to view the file at all. A permissions (or owner) password lets anyone view it but is meant to block printing, copying or editing, though many readers do not strictly enforce that restriction.
Will adding a password change the PDF's content or formatting?
No. Password protection encrypts the existing file; it does not alter the text, images or layout, only who can open or use it.