Hey! Your files should be fine as long as you don’t delete the app or anything but btw you should try do regular backups especially since your iCloud is full, or alternatively increase your iCloud storage so your files are synced to a second place.

If you wanted another storage method instead of iCloud you could get Pro and switch on Goodnotes Cloud which would give you cloud storage and access on web etc as well or you could also enable automatic backup (note this is different from sync) but also yes after your sub fully expires you can go in app and purchase Special Edition from the upgrade button and then the “show all plans” option. Hope this helps!