PDF Encryption
Protecting a PDF with a password, typically using AES-128 or AES-256 encryption, so the file cannot be opened or modified without the key.
PDF encryption protects a file with a password. When enabled, the PDF cannot be opened, edited, or printed without the correct password — the content is mathematically scrambled using a strong cipher.
Two kinds of passwords
The PDF format supports two password types:
- User password (open password). Required to open and view the PDF at all. Without it, the content is inaccessible.
- Owner password (permissions password). Allows the file to be opened without a password, but restricts actions like printing, editing, or copying text. The document appears normally but with reduced capabilities.
Encryption strength
Modern PDF encryption uses AES (Advanced Encryption Standard) with either 128-bit or 256-bit keys. Both are considered cryptographically strong — a well-chosen password is essentially unbreakable with current technology. Older PDF encryption (RC4, 40-bit) is weak and can be cracked in minutes; avoid it.
Caveats
- A strong password is non-negotiable. AES-256 with the password "1234" is no more secure than no encryption at all.
- Encryption protects the file at rest. Once you open and share the decrypted content, the protection is gone.
- Owner-password restrictions (no printing, no copy) are enforced by polite PDF viewers, not cryptographically. They can be bypassed.
Tools
- Protect PDF adds AES-256 password encryption to a PDF
- Unlock PDF removes the open password from a PDF you own (you must know the existing password)