On some (not all) PDF documents: when I edit with Apple Pencil 2, the eraser doesn’t work, but other editing tools do. On these documents, I also then get a “Couldn’t save document” message. However, the un-erased edits do seem to have been saved when I reopen the document.
Think I may have found the answer to my question. Two of my troublesome pdf files were “Shared” to DTTG from Bookends with its “Embed Annotations” option. Sharing them using the Bookends “Flatten Annotation” option enabled these files to be fully editable within DTTG. Another pdf file may have been shared to DTTG in a locked format.
Thanks for the follow-up.