It is a bug with the Preview app when a person edits a PDF file that has restrictions enabled on it. Even if the PDF has a fillable form the Preview bug enables the restriction to also cover the entire PDF file instead of just the core document. You need to download a clean copy of the PDF file and use some other PDF editor to modify the file. Do not let the Preview app modify the PDF in any manner or the PDF file will become locked again.
Read all of @etresoft's posts in the following thread for more information:
Unwanted PW Protection on PDF - Apple Community
You can provide Apple with product feedback here (Apple won't respond):
Feedback - macOS - Apple