คู่มือความปลอดภัย PDF — การเข้ารหัสและความเป็นส่วนตัว

PDFs have two separate password slots:

**User password (Document Open password):** Required to open the document. Anyone without this password sees a blank page or an error in their PDF viewer.

**Owner password (Permissions password):** Controls what an authorised reader can do — printing, copying text, editing. This is weaker than it sounds (see below).

You can set both on the same document, one, or neither. Most 'password protect PDF' tools set the User password by default.

When you set a User password, the PDF is encrypted (typically AES-256 for modern PDFs). The file contents are mathematically scrambled and cannot be read without the correct password. This is robust — brute-forcing a strong AES-256 password is not practical with current hardware.

Caution: PDF encryption protects the file at rest. Once a recipient opens the PDF with the correct password, they have access to the decrypted content on their device. PDF encryption does not prevent an authorised reader from screenshotting or printing.

Owner password (permissions) restrictions — 'no printing', 'no copying', 'no editing' — are enforced only by well-behaved PDF viewers. Any software that does not respect these flags can ignore them. This includes free tools, scripts, and even some popular PDF readers.

Conclusion: Use permissions restrictions to signal intent ('this PDF should not be edited'), but do not rely on them to technically prevent a determined person from copying or printing content.

A watermark does not prevent copying or editing — it is a visible deterrent and a way to identify leaked copies. Use watermarks when you want to:

- Attribute a document to a specific recipient (e.g. 'Confidential: John Smith') - Mark draft versions clearly - Protect brand assets shared for review

Watermarks and password protection can be combined: encrypt the file AND add a watermark for layered protection.

1. **Set a strong User password** (12+ characters, mixed case, numbers, symbols) 2. **Share the password through a separate channel** — never in the same email as the PDF 3. **Set an expiry if possible** — some document management systems support link-based access with expiry 4. **Flatten forms before sharing** — if the PDF contains editable fields, flatten them to prevent modification 5. **Remove metadata** — PDF metadata can contain author name, creation software, and revision history