Remember, you cannot trust whether the document has any restrictions on it once Preview has handled the document as Preview may have twisted them around.
If you download a fresh clean copy of the PDF from the original source, then you better trust what Adobe is reporting about the PDF.
Here is another thread regarding this issue....please read all of @etresoft's posts (there are several of them by @etresoft all with extra little bits of information):
https://discussions.apple.com/thread/255437832?answerId=260120182022&sortBy=oldest_first#260120182022
@etresoft's last post in that thread suggests it may be possible to use "File --> Revert" to put the PDF file back to its original condition if you cannot re-download a clean copy. This worked for @etresoft with one test PDF file.
Also within that thread it seems that this issue comes up for a password protected PDF file with a fillable form. When Preview modifies a PDF of this type of by filling in the form, Preview corrupts the file making the password required to even open it. The original password protection was only meant as a restriction to keep anyone but the original author the ability to modify the main document to preserve the document's integrity. The restriction was not meant to apply to the fillable fields portion of the document.
Edit: Actually the issue I described in the last paragraph was from the following post by @etresoft in another thread
https://discussions.apple.com/thread/255083223?answerId=259458189022#259458189022
