OK, here is what I found on Reddit, which seems to have taken over from here, mostly. In case anyone else has this problem.
This worked - open the PDF on my computer, then Print as PDF, then can send that and write on it.
An extra step, which will be easy to incorporate into my daily routine.
There were various other suggestions in Reddit threads that did not work. Hope this saves someone (future me, probably!
) some time.
I notice that when the PDF is writeable, it either lists the author if there is one, or has Unknown as author. When it is not writeable on, the space where author or Unknown would be is blank.
I'm glad I can still use my Scribe for this - I also keep my invoicing records and project details in a PDF planner on it, so would be quite lost if they stopped allowing this functionality.