If you tap New... > "Import" from the library view and try to browse for a supported file on a cloud storage like OneDrive, you might find that no files are listed, like this message:
Judging from this discussion, it's likely a bug related to the integration of Files and that cloud storage because the Files app itself, as well as any other app using the Files interface, has this issue. As a workaround, we recommend importing files from that cloud storage via Split View instead:
1. Make sure the cloud storage app (like OneDrive or Dropbox) is on the Dock.
2. Open GoodNotes, and open the Documents tab.
3. Swipe up from the bottom of the screen to open the Dock.
4. On the Dock, touch and hold the cloud storage app, then drag it off the dock to the left or right edge of the screen.
5. Drag a file from the cloud storage app to GoodNotes Documents to import that file. To import multiple files, while holding one of them on the other window, tap (with another finger) to add the remaining files to the selection, then release your finger.
Alternatively, you can open the cloud storage app and share a file with GoodNotes via the Share button or a similar command.