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.

You might recover an earlier copy of the file from your Time Machine backup drive and see if that is also affected.

Meanwhile, have you restarted your Mac lately? Rebooting is the best first step in troubleshooting.

If a simple reboot of your Mac doesn’t fix things, see if the problem still happens in safe mode.

How to use safe mode on your Mac - Apple Support 

Safe mode will often correct weird software behavior by forcing the OS to do disk repair, clear caches and other housekeeping and temporarily disables third-party mods.

Can you open the file while in Safe mode?

You can exit safe mode by restarting your Mac normally, then reevaluate the problem.

To expand upon @BDAqua's post......if you use a PDF with fillable forms, then you may encounter a nasty bug where the entire PDF will become password protected instead of just the main portion of the PDF that the creator password protected to prevent the PDF from being modified except for the fillable form entries.

I have seen some people report being able to lose the password protection to return the PDF to its original state, but I don't recall the method(s). Search the macOS Sequoia forums since there are many posts there which mention the option(s).

Or download the PDF from the source again, but this time use another PDF app to fill in the form and avoid opening that PDF within the Preview app.

mrstee wrote:

Update-I was able to find the document using Time Machine. It's a weird document though. When I open it in Preview and show Thumbnails there is nothing in the Thumbnails. Well, it's not the usual looking PDF in Thumbnails... And when I try to print it there is one page which is blank!

Pages can't open it.

Wondershare PDFelement can open it but there is nothing in the Thumbnails.

TextEdit just shows gibberish.

It's a typed document. A poor copy. It's over 200 pages long.

Sooo.... This whole issue is weirder and weirder!

How was the PDF file created? You said it's "a poor copy," is it something that was scanned in to create a PDF?

You can also try opening it in Adobe Acrobat Reader, which is free to download, install and use.

The password protection on PDFs is abysmal. No matter what encryption depth you use (in apps where you can select it), they are easily bypassed. Just look for "unlock pdf" as a web search. There are easily dozens of sites where you can upload a password protected PDF and it will remove the lock.

There was also a cranky bug in Preview that would lock a .pdf when saving, might see if adobe Reader can en it.

"Locked" is NOT the same as "Password Protected".

Unfortunately, if the file is password protected and you can't remember the password, it is probably not possible to open the file.

I think Preview sometimes automatically locks PDF files. I have encountered this before, but I have no idea at all what the password is.

If your PDF was already password protected and without knowing the password, the fastest way is to use a password remover such as cisdem, lostmypass.

Click once on the document to select it and bring up it's Info pane

In the Info pane at the top will be a check box that says locked. Uncheck it. That check box just makes the file read only and uneditable.

Try using Spotlight > Keychain access which may be of assistance

Thanks!:)