I use 1Password1 to store not only my own information, but also my sisters home Wi-Fi information.

Since she does not have a 1Password licence, I printed out the information for her to store in a safe place. Now, I was just thinking of using the printing dialog (under "Security Options...") to create a "password protected" PDF.

But what does that actually mean and how secure is it?

  • Does this encrypt the whole PDF?
  • What encryption is used?
  • Is it compatible with Windows and Ubuntu?

There does not2 seem to be a knowledge-base article on support.apple.com. The info dialog in Finder merely says that the PDF is "Password Encrypted".

enter image description here

3 Answers 3


The last Time I checked a 128 bit RC4 encryption of the whole document was used. Also, compatability should be vested.

Here's a screenshot of the output of GuaPDF: enter image description here

  • The download on the GuaPDF website was reported by Sophos AV to contain a virus "Slenfbot-E". Hence, I removed your link and cannot verify that 128-bit RC4 encryption is used. Do you mind creating a password protected PDF in Preview and checking it for yourself?
    – gentmatt
    Jan 12, 2013 at 12:50
  • So here's a screenshot. I encrypted the PDF with checking all check boxes in the Security Options. You have to decide if you trust the output of this application.
    – cyphorious
    Jan 12, 2013 at 17:21
  • Thanks for the update! I would always prefer first hand documentation by Apple about their software, but I'm willing trust this vendor for now ;)
    – gentmatt
    Jan 12, 2013 at 18:02
  • 1
    Just an update regarding current versions of Preview.app here in 2016. The version of Preview.app that comes with Mac OS X 10.11.6 still encrypts PDFs with 128 bit RC4 encryption.
    – jefe2000
    Aug 24, 2016 at 17:23

But what does that actually mean and how secure is it?
Does this encrypt the whole PDF?
Is it compatible with Windows and Ubuntu?

Part of the PDF specification describes the encryption of PDF data, and Preview produces a 'to-spec' encrypted PDF.

Encrypted PDFs made by Preview are valid PDFs that can be opened (when the password is provided) by any PDF app that follows the spec on any platform.

The level of encryption depends on the PDF version.

Early versions of MacOS saved with PDF v1.4, giving RCA-128bit encryptions. Newer versions save as PDF v1.6, giving AES-128 encryption. You can see the version level of the PDF document in Get Info.

  • What does the PDF specification say about the encryption algorithm to be used? RC4 is rather weak nowadays, are there alternatives listed there?
    – nohillside
    Sep 5, 2020 at 13:24
  • @nohillside Since PDF v1.6 (Acrobat 7), AES-128 has been used. Saving an encrypted PDF in Preview (Mojave) produces a v1.6 PDF, (when the document would otherwise be saved as v1.4), so I presume that's what MacOS uses.
    – benwiggy
    Sep 5, 2020 at 13:30

Another method, guaranteeing 256-bit AES encryption, is to use Disk Utility to create an encrypted disk image of a folder containing the PDF, or just a text file for that matter. One cannot create a disk image of a file - it must be a folder.


  1. Create a folder, place file(s) to encrypt within

  2. Open Applications/Utilities/Disk Utility.app

  3. Menu item File > New > Disk Image from Folder

  4. Choose folder to encrypt

  5. Click the Image button

  6. Select .dmg save location, read-only and 256-bit AES Encryption:

    enter image description here

  7. Click the Save button, then enter a password of your choosing or use the 'Password Assistant' which can generate a password for you of several types and lengths. You are also able to store this password in your Keychain.

Naturally, the encrypted .dmg file can only be opened on a Macintosh.

This technique is handy for encrypting anything that can be put in a folder.

  • 1
    Thanks, I'm already aware of this nice and simple to use tool in Disk Utility. But I really want to secure the PDFs themselves because I cannot control if my sister's lax security practices will make her to simply copy the PDF from the encrypted disk image to somewhere else.
    – gentmatt
    Jan 13, 2013 at 7:39

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