PDF 安全指南 — 加密与隐私
PDF 支持多种安全功能 — 密码保护、加密、权限限制和数字签名。每种功能的作用各不相同，了解这些差异有助于你根据自身情况选择合适的保护措施。
两种 PDF 密码类型
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.
PDF 加密的实际作用
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.
权限限制无法阻止什么
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.
水印与密码保护的区别
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.
分享敏感 PDF 的最佳实践
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
常见问题
- PDF 密码保护有多强？
- 如果你使用强密码，它非常强大。现代 PDF 采用 AES-256 加密，在任何现实的时间范围内，都无法通过暴力破解包含 12 个以上随机字符的密码。弱密码（字典词、简短字符串）则很容易被现成的工具破解。
- 没有密码可以解锁受密码保护的 PDF 吗？
- 对于用户密码（加密），基本上不能 — 无法在采用 AES-256 和强密码的情况下破解。对于所有者密码（仅限权限），可以 — 许多工具可以在不知道所有者密码的情况下移除权限限制，因为文件内容本身并未被加密。
- WhiskFile 会存储我的密码吗？
- 不会。WhiskFile 在内存中处理你的 PDF，应用密码保护，然后返回加密文件。密码不会被存储、记录，也不会用于加密文件以外的任何用途。文件将在 1 小时内删除。
- 为了符合 GDPR 合规性，我应该使用 PDF 密码保护吗？
- PDF 密码保护可以作为数据保护方法的一个环节，但仅凭它并不足以完全符合 GDPR 合规要求。静态和传输中文件的加密、访问控制以及数据保留策略都是合规工作流的一部分。