Invisible Watermarks and Other Ways to Protect Your PDFs

PDFs are a fantastic way to share documents – they preserve formatting, are widely compatible, and can even be compressed for easy transmission. But what if that PDF contains sensitive or monetized information? The Portable Document Format (PDF) offers some security, which when stacked can be quite effective. These include:

  • Watermarks. Adding content to a PDF, especially content customized to the end-user (such as their name and other personally-identifying information like phone/email, can not only personalize a PDF, but also thwart unauthorized sharing. Keep reading to find out how to make your watermarks most effective against PDF theft.
  • Encryption. Arcfour RC4, and the superior 128-bit key or 256-bit AES encryption
  • Passwording. User and owner passwords can be set. An owner password gives the holder full control over the PDF, including the ability to remove passwording entirely — share with caution! Passwording requires encryption of the PDF file.
  • Redaction. pdfSweep removes sensitive information entirely from PDFs (but what if you want the paying end-user to be able to see that information)

    Watermarks: A Visual Deterrent

    A watermark is usually a semi-transparent image or text stamped onto the background of each page in a document. The watermark can be your company logo, a confidentiality notice, or even a name or # copies authorized, or a copyright notice. While watermarks can’t completely prevent copying, they do make unauthorized distribution visually unappealing.

    Watermarks can be removed from PDFs. But what if the end-user cannot see the watermark, and doesn’t know it is there? Invisible watermarks are a great weapon in the fight against PDF piracy. We recommend a very small, transparent line of text reading out the customer’s personally-identifying information, hidden somewhere — anywhere — on the page.

    Our WordPress plugins TCPDF Bridge, WaterWoo, and EDDiMark all allow Watermarks in any transparency: opaque to 100% invisibly transparent. Since our plugins allow more than one watermark location, we recommend having one visible watermark, and one invisible watermark. Thieves can and likely will remove the visible watermark, and not realize there is a second watermark in place. Maybe suggest to your customer that there are many hidden watermarks. Paranoia goes a long way toward discouraging theft…

    Password Protection: A First Line of Defense

    A password encrypts the PDF content, making it unreadable without the correct password. This is essential for documents containing confidential information. Every PDF has two passwords:

    • Open (User) Password: This restricts access to viewing the PDF altogether.
    • Permissions (Owner) Password: This allows the PDF to be opened with a password, and furthermore allows to opener to edit/remove PDF permissions and passwording altogether.

    Remember: Strong passwords are key! Use a combination of upper and lowercase letters, numbers, and symbols. Avoid using easily guessable words or personal information. Our plugins allow for creating a static password, using customer email as the password, or even customizing passwords entirely using WP filter hooks.

    As mentioned, encryption is required in order to password PDFs. If your webhost’s server CPU can support the load of AES encryption, then we strongly recommend you use that. RC4 is not a secure encryption method, but oftentimes shared hosting servers can only handle RC4.

    Combining Watermarks and Passwords

    For higher protection, use watermarks and passwords together. The password thwarts unauthorized access, while the watermark discourages further distribution. This is a great strategy for highly sensitive documents, contracts, or proprietary information.

    Encryption using our plugins allows for more than passwording, though. Once encrypted, PDF permissions can be edited, for example removing the ability for the end user to copy, print, and modify your PDF. (Keep in mind that if someone has the owner password, these permissions can be changed.)

    Limitations to Consider

    While effective, watermarks and passwords aren’t foolproof. Tech-savvy and other determined individuals may be able to remove watermarks or bypass weak passwords. Additionally, some free PDF readers might even not recognize password or permission restrictions. Again, we recommend hiding your watermarks by making them small and invisible.

    The Takeaway

    Watermarks and passwords are a strong combination for safeguarding your sensitive PDFs. By implementing these methods, you can deter most casual copying and unauthorized access, ensuring your PDF information stays secure.

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