How can I unlock a locked document without knowing the password?

mrstee wrote:

What really puzzles me is that when I get info on the document, the box for Locked is NOT checked (see screenshot).

That looks like a password protected document, not a "locked" one. Password protection requires multiple specific steps when saving, including selecting the save with password option, and entering the password twice. I can't imagine how that could have happened "accidentally."

As others suggested, try opening the file with other software. Including Adobe Acrobat (or Acrobat Reader), and even Ms-Word can open pdf files. If they all request a password then it appears a password is needed. What might be a quick password you might have used on the fly (don't answer that here, but try entering it).

PDF password protection is pretty strong so I don't know a straightforward way to get around it.