I actually did contact them. This was their response. I’m pasting it here in case others have the same issue.
I was able to make changes to PDFs directly and successfully save into DTTG using the app PDF Reader Pro. I prefer the UI of PDF Expert, but at least, I have a workaround.
Thank you for reaching out to us and please accept our sincere apologies for any inconvenience or confusion that took place.
I’m glad to tell you that we’ve managed to reproduce the issue in question (when the integration doesn’t work correctly between PDF Expert and DEVONthink To Go) and created an entry in our bug-tracking system. Our developers are doing their best in order to fix it shortly. However, it may take a while, but once we have any news, we’ll follow you up.
We are extremely sorry for the current inconvenience and hope for your understanding.
Meanwhile, if you have any additional questions or requests concerning the product improvement, please do not hesitate to contact me. I’ll be happy to help.
Have a wonderful and sunny week ahead!