Unable to unlock a downloaded document/Unable to unlock a preview pdf/asking for password that was never set

What version of macOS are you using? And can you provide a URL so that we can download the same PDF and examine it?

There was a really nasty bug in Apple's Preview that would cause any fill-in PDF to get locked as soon as you entered any value and closed the document. Here are a couple of references:

Editable PDF is automatically encrypted - Apple Community

Preview is automatically assigning passwo… - Apple Community

However, I haven't seen anyone report this problem for a while. I also can't reproduce it on macOS Monterey 12.6.2. But since I can't reproduce it, I can't tell if it was fixed. If you can provide a PDF that shows the problem, I can tell you if it is the same problem or not.

Joerel Ramos wrote:

I've never set up a password for Preview. When I wanted to mark a file I downloaded ie. a credit card statement, it keeps on asking for an owner password to unlock it. I used my computer log in and my apple ID log in as well as the credit card company's password but they wont unlock the document.

Seems an issue with your provider of your "a credit card statement"...

talk to your associated bank if you have an issue here.

Alternatively— try viewing the "statement on-line and compare your results.

Joerel Ramos wrote:

I think it is the same problem.

But that problem no longer exists. As far as I can tell, it has been fixed.

Whoever creates a PDF can assign a number of different permissions to it. It sounds like your particular document simply doesn't allow modifications.

To verify that the pdf is at fault and not Preview download and try the free PDF reader app Skim. If it is required to enter a password then the problem most likely lies in the pdf file.