I honestly have no idea. I've never used Apple products myself. (As a Linux user, it would be somewhat challenging anyway...)
But the only problem with Google Drive is that Google somehow decided it was
okay to modify the filenames of stored files.
Apparently that makes sense to them, because who would ever be stupid enough to put numbers in parentheses in a filename, right
So Google decided that was how they would implement a versioning system.
Given that Google Drive is the only service I have heard of so far with that (ridiculous) problem, iCloud is
probably okay. But in order to
know one way or another, I would have to have used iCloud... and I haven't, so I'm not going to promise anything.
...
If you do store your library in iCloud, and Goodreader can open the PDF files directly and write the modified PDF back to iCloud, then chances are you won't even have to export the PDF to the computer and update it inside your library. The iCloud sync should do all the work for you.