"Save a Copy" not always possible

I need to add notes to every Purchase Order that comes to us, and sometimes they come with a digital signature.  When I try to edit these I get an alert telling me that it cannot be edited because of the digital signature.

Most times it just says that, and that's it.  But infrequently it will suggest that I "Save a Copy," which is excellent because "Save a Copy" magically appears in the File drop-down menu.  But when it's not suggested "Save a Copy" is not available.  I can't find any rhyme or reason why sometimes it is possible and sometimes it is not.  I'd love to be able to stop having to print them, scan them, and edit the scanned version, which is basically what "Save a Copy" is doing for me.

Is there a way to have "Save a Copy" available at all times?