PDF Security, Honestly: Passwords, Permissions and Real Redaction

PDF has several features that sound like security and provide very different levels of it. Confusing them is how confidential text ends up published under a black box that anyone can copy and paste out of.

User passwords: real encryption

A user password (open password) means the file's contents are encrypted. With AES-256 and a strong password, this is genuine protection — without the password the bytes are noise. This is the only PDF security feature that stands up to a determined attacker.

The caveats are the usual ones for any encryption: the strength is the password, not the algorithm. A six-character password on AES-256 is a six-character password. And if you email the file and the password in the same thread, you have encrypted nothing.

Owner passwords and permissions: not security

Permission flags — no printing, no copying, no editing — are requests. The file is readable; the flags simply ask compliant software to restrict what it offers the user. Any non-compliant reader ignores them entirely. Use permissions to prevent accidents and signal intent, never to protect a secret.

Redaction: the one people get wrong

Drawing a black rectangle over text adds a black rectangle. The text is still there, underneath, fully selectable and fully searchable. This exact mistake has produced court filings, government disclosures and corporate documents where the confidential content was recoverable by pressing Ctrl+A.

Real redaction removes the underlying content objects and then draws the marking. After redacting, verify by selecting all text on the page and pasting it somewhere plain — if the redacted string appears, the file is not redacted.

The metadata nobody checks

• Author, company and title fields, often left from the original template's owner.
• Revision history in files exported from word processors with tracked changes enabled.
• Embedded thumbnails generated before the redaction was applied.
• Attached files and embedded objects that are not visible on any page.
• Layers (OCGs) that are hidden but present — hiding a layer is not removing it.

A pre-release checklist for sensitive documents

1. Apply true redaction, not annotation.
2. Select-all and paste into a text editor; confirm no redacted content appears.
3. Strip document metadata and set fresh title/author values.
4. Flatten layers and annotations.
5. Regenerate the file so thumbnails and cached previews are rebuilt from the redacted content.
6. If it must be encrypted, use AES-256 with a long passphrase and deliver the passphrase out of band.

Where browser-side processing helps

For genuinely sensitive documents, the strongest privacy property is that the file never leaves your machine. Tools that run in the browser via WebAssembly do the work locally — there is no upload to intercept, no server copy to subpoena, and nothing to retain. That is a structural guarantee rather than a policy promise, and for legal and medical documents it is worth insisting on.