The new iPadOS introduces some improvements to Share Sheet and makes it a "Smarter share sheet" which is a new way to quickly share content from one app to another app faster. However, there are cases the destination apps won't show up in Share Sheet because users haven't used certain apps yet for the iOS to learn your usage patterns.
To address the issue with GoodNotes doesn't appear in Share Sheet's action list when users import supported files (PDF/PowerPoint/Word) from another app, on the second row of the Share Sheet in which you'd see a list of actions, try to scroll to the right, tap "..." ("More" button), this brings up the "App List", tap "Edit" and make sure you tap "+" button in green color to add GoodNotes 5 to the default app for handling PDF files.
You'll always see "Copy to GoodNotes" option when opening a supported document and share via iOS/iPadOS Share Sheet from now on.
Using GoodNotes on a device managed by an organization (University, school, company)
In some cases, GoodNotes may not appear in the Share Sheet because the organization that manages the device you're using hasn't enabled third-party app access. Please contact your IT department and ask them to verify or enable GoodNotes as an app that can accept and share documents from/with other apps.