I have uploaded a pdf and this all works well. Thank you. Agt the end of the day, I exported a backup choosing the JSON format. I then imported the resulting zip - as I always do, to my offline knowledge base in Remote Desktop for Mac - latest version. The pdf was reported as missing Rem.
I think this its a bug, as the export - import is incomplete.
I can now confirm that after the latest update, which I downloaded today on my desktop Mac version, that the PDF’s are exported and imported correctly. It also correctly imports the highlighted areas. I am very happy about this! Thank you so much for addressing this. I now feel that the desktop version is much more compatible to the online version and is well on its way to feel Mac-like. Thank you!
Thank you for your patience and assistance 
Closing the topic now!