能否通过对比打开权限受保护PDF与解密版PDF提取密码？

Short answer: No, you can't extract the password directly by comparing the encrypted (80MB) and decrypted (76MB) PDF files. Here's why:

• PDF encryption doesn’t store the password anywhere in the file. Instead, the password is used to generate an encryption key via a key derivation function specific to PDF standards, which combines the password with file-specific data. This key encrypts the PDF’s content streams, but the password itself isn’t present in either file in a recoverable form.
• The size difference (80MB vs 76MB) comes from extra metadata in the encrypted file: things like encryption settings (algorithm used, permissions, key length), padding bytes added to align content for block encryption (like AES), and structural overhead to support the encryption workflow. None of this extra data contains the password.
• Comparing the two files might reveal which parts of the PDF were encrypted (mostly content streams, not structural metadata), but that doesn’t help reverse-engineer the password. The encryption process is intentionally one-way—you can’t derive the password from the encrypted content or key without brute-forcing millions of combinations, which comparing files doesn’t simplify.

If you need to recover the password, your only practical options are brute-force attacks or dictionary attacks using common password lists—but comparing the two PDFs won’t speed up or assist this process.

内容的提问来源于stack exchange，提问作者Joseph