If you need to email a highly sensitive document—such as a tax return, a medical record, or a signed legal contract—you should never send it as a standard, unprotected PDF file. If the email is intercepted, or if you accidentally type the wrong email address into the “To” field, anyone can open and read the file. To secure sensitive information before transmitting it over the internet, you can use the built-in macOS Preview application to encrypt the PDF with a robust, uncrackable password.
How to Encrypt a PDF with a Password
The macOS Preview app utilizes advanced 128-bit RC4 encryption (and newer AES standards depending on your macOS version) to lock the file. This means the password is not just a polite request; without it, the file’s binary data is completely scrambled and mathematically impossible to read.
- Double-click your sensitive PDF file to open it in Preview.
- Look at the top menu bar (next to the Apple logo) and click File.
- From the drop-down menu, select Export… (Do not select “Export as PDF”).
- A save dialog box will appear. Change the file name slightly (e.g., add the word “SECURE” to the end) so you do not accidentally overwrite your original, unencrypted master copy.
- Look at the bottom of the dialog box. Check the small box labeled Encrypt.
- Two new text fields will instantly appear. Type your chosen password into the Password field, and then carefully type it again into the Verify field to confirm there are no typos.
- Click the Save button.
Preview will immediately generate a brand new, fully encrypted copy of the PDF on your hard drive.
How to Open a Password-Protected PDF
When the recipient receives the encrypted file and attempts to open it, they will not see the document. Instead, their PDF viewer (whether it is Preview on a Mac, Adobe Acrobat on Windows, or even a web browser like Chrome) will halt the process and display a blank, grey screen with a single password prompt.
They must type the exact password you created. If they type it correctly, the software will decrypt the data on the fly and display the document normally.
Crucial Security Warning: Never send the password to the recipient in the exact same email as the encrypted file. If a hacker intercepts the email, they will instantly possess both the lock and the key. Always send the password via a separate, secure communication channel, such as a temporary Signal message or an SMS text message to the recipient’s personal phone.