What Unlock PDF does to an encrypted file

Which kinds of encryption it can remove

PDF has had several generations of password encryption, and a file keeps whichever one the program that made it chose. Unlock PDF handles the four used by the standard password system:

• RC4 40-bit, found in files from very early PDF software.
• RC4 128-bit, used by older software and some document systems.
• AES 128-bit.
• AES 256-bit, the strongest of the four and the default in the Protect PDF tool.

When you add a file, the tool reads its encryption settings and shows the method and the list of restricted actions (printing, copying, editing, commenting, filling forms, and inserting or deleting pages). Anything outside those four methods gets the message "This PDF uses an encryption method that can't be removed yet". Files locked to a digital certificate are refused for a different reason: there's no password that would open them.

What happens to the file

The password is used to work out the file's encryption key. The tool then decrypts every encrypted string and stream in the file, deletes the encryption dictionary, and saves the result as a new copy. Nothing else is rebuilt. The pages, bookmarks, form fields, comments and attachments are the same objects as before, just no longer scrambled. If the original declared its metadata as unencrypted, that part is left exactly as it was.

For a file with restrictions but no open password, the tool tries an empty open password, which is how a reader displays such files in the first place. That's why no typing is needed.

Passwords that don't match when you're sure they're right

The tool tries your password in more than one text encoding, because different programs store passwords differently. With AES 256-bit files, accented letters and non-Latin scripts work, since that format stores the password as Unicode. Older RC4 and AES 128-bit files only support a limited character set, so a password with unusual characters may have been stored in a way that varies between programs.

Some more ordinary causes:

• A space copied along with the password from an email. Spaces count, at the start and the end too.
• Caps Lock, or a keyboard set to a different layout, which swaps letters like Y and Z or moves symbols around.
• Letters and digits that look alike in an email font: a capital O and a zero, a lower-case l and a 1.

Use the show password button to see exactly what you've typed.

Digital signatures and other side effects

Removing encryption means writing out a new file, and that matters for signed documents. A digital signature covers the exact bytes of the file it was applied to. Once those bytes change, signature checkers in apps like Adobe Acrobat Reader will report that the document was modified after signing, even though the visible content is identical. If you need the signature to verify, keep the protected original alongside your new copy.

You also don't always need this tool first. Most EddiesPDF tools ask for the password when you add a protected file and decrypt it in your browser, and the files they save don't carry a password. To merge three protected statements, for example, you can go straight to Merge PDF.