Backup to Cloud Services like OneDrive works fine.
However, I have not found a solution to transfer files easily from PC to GoodNotes on my iPad. E.g. if I want to read and annotate a PDF in GoodNotes.
It would be very simple to achieve this by a two-way synch with OneDrive so that files that are copied to the OneDrive GoodNotes Folder on any device can be read and edited on the iPad. I don't think I will continue to use GoodNotes without such a feature.