PDF Password Protected

It is a bug with the Preview app. The original document likely had an editing password so the core document could not be changed, but would allow the ability to fill in the form (if included). The Preview bug tends to activate the limited password restrictions to the whole document. There are lots of other threads on this forum recently about the issue (seems to mostly occur with Sonoma, but older versions of macOS will see this occur sometimes as well).

Download a fresh copy of the original PDF and use a different PDF app to fill in the form. If you let Preview touch it again, the new copy will become locked as well.

You can provide Apple with product feedback here:

Product Feedback - Apple

To get rid of the password protect, when you generate the file go>print>save as pdf. Then it won't be password protected and you can email the file if you need to. I have used Macs since 1992, and this automatic password protect is the stupidest idea they have ever come up with.