Sending sensitive documents—such as tax returns, bank statements, or legal contracts—via email or cloud storage carries a significant security risk. If an unauthorised person gains access to your inbox or intercepts the file, your private information is immediately exposed. The most effective way to prevent this is to securely password-protect your PDF documents before you share them.
Fortunately, if you are using a Mac, you do not need to purchase an expensive Adobe Acrobat subscription or upload your private documents to a sketchy, third-party online converter. Apple has built robust, military-grade PDF encryption directly into Preview, the default image and document viewer included with every version of macOS.
Why Use Apple Preview for PDF Security?
Preview uses strong RC4 or AES encryption (depending on your macOS version) to lock the PDF. When you apply a password using this method, the document is mathematically scrambled. It is not simply a “hidden” layer that can be easily bypassed; without the correct password, the document cannot be read by anyone, regardless of what software or operating system they are using.
Furthermore, because Preview processes the document entirely locally on your Mac’s hard drive, your sensitive data never leaves your computer, ensuring maximum privacy.
Step-by-Step: How to Add a Password to a PDF
To encrypt your document, follow these exact steps:
- Locate the PDF file on your Mac (e.g., on your Desktop or in your Documents folder).
- Double-click the file. Unless you have changed your default settings, it will automatically open in the Preview application.
- In the top menu bar, click on File, and then select Export… from the drop-down menu. (Do not choose “Export as PDF” or “Save”, as these options will not provide the security settings).
- A new dialogue box will appear asking you where you want to save the new file.
- At the bottom of this dialogue box, you will see a small checkbox labelled Encrypt. Check this box.
- Two new text fields will appear: Password and Verify.
- Type a strong, unique password into both fields. Ensure it is something you can remember or securely communicate to the intended recipient.
- Click the Save button.
Preview has now created a brand-new, encrypted copy of your PDF. If you try to open this new file, macOS will immediately prompt you to enter the password before displaying any content.
How to Remove a Password from a PDF
If you have received an encrypted PDF and you want to save an unlocked copy for your own records so you do not have to type the password every time, you can also use Preview to strip the encryption away.
- Double-click the encrypted PDF file to open it in Preview.
- Enter the correct password to unlock the document.
- In the top menu bar, click File, then select Export….
- In the export dialogue box, ensure the Encrypt checkbox is un-checked.
- Click Save to create a new, permanently unlocked copy of the document.
Best Practices for Sending Encrypted PDFs
While the PDF itself is now secure, the way you share the password is equally critical. Never send the password in the same email or text message as the attached PDF document. If a hacker intercepts the email, they will have both the lock and the key.
Instead, use an alternative communication channel. If you email the PDF, send the password via an encrypted messaging app (like Signal or iMessage), or simply call the recipient on the phone to provide the password verbally.