If you have downloaded GoodNotes 5 and upgraded to the full version on one device and you would like to use it on another, you can simply redownload GoodNotes 5 from the App Store or your Purchase History. As long as your other Apple devices (iPads, iPhones, Macs) are logged in with the same Apple ID you used for the original GoodNotes 5 purchase/download, you will be able to use the app on the other devices.
Downgrade
To do this, you will first need to download the Goodnotes 6 app from the App Store and downgrade it to GoodNotes 5. So long as the app recognises you had previously purchased GoodNotes 5, you will be able to downgrade. If you had not purchased GoodNotes 5, then you will no longer be able to do so and will need to use the free version of Goodnotes 6 instead.
- Tap the ⚙️ icon in the top-right corner of the Library view > Settings > Downgrade.
- Disable "Run Goodnotes in Version 6." The app will exit.
- Relaunch the app.
If you are a paying user and experience difficulties in downgrading to GoodNotes 5 and unlocking the full app on second device, see here: I can't restore my purchase