Can't modify or annotate PDF files with Preview.app

I am using macOS 10.15.3 (Catalina) on a MacBook Pro with its current Preview application. I cannot save a PDF that I have modified or annotated. Here's what I have seen:

Open a PDF, use Tools -> Annotate -> Signature to add my previously-saved signature. The signature appears as expected, but I see the following:

• the document name changes to the original document name followed by "copy"

• this warning (below) that says, "The original document can't be changed, so a duplicate with your changes has been created."

• and a message "...this document could not be autosaved."

Next I choose File -> Save, and see this message:

The document "original-document-name copy" could not be saved as "original-document-name copy.pdf"

I was able to File -> Export as PDF... and save the file.

Is this the normal/expected behavior? Thanks.