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 both the cloud storage app (like OneDrive or Dropbox) and GoodNotes are 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, tap and hold one of them and release to select it, tap to select the remaining files, then drag the selection to GoodNotes Documents.