Here's how to make sure your GoodNotes app has a proper iCloud sync set up and syncing. Once the information is stored on iCloud, it's easy to retrieve or download your GoodNotes app data from any device later.
You can setup iCloud sync in order to sync GoodNotes documents between iPhones/iPads/Macs. If one of your iPad is damaged or you lost the access and you are unsure whether the data has been synced or not, then you can quickly verify whether sync status on one of your devices using the same Apple ID this way: open iPhone/iPad's Settings > your Apple account (top row in the Settings list) > iCloud > Manage Storage > look for GoodNotes and verify if your app data is saved there?
If the iCloud sync wasn't activated in GoodNotes, the next thing you can do is to check whether you have iCloud's Device Backup enabled as there could still be a chance the entire app data has been backed up via the device backup (refer to this article from Apple). Just open iPad's Settings > your Apple ID > iCloud > iCloud Backup and verify if it's ON. Then, tap "Manage Storage" > iCloud Backups > verify whether your device is listed out there in Backup section? Next, follow the steps as described on the link above to restore your entire device from an iCloud device backup.
To protect your important notebooks better, we highly recommend users to turn on iCloud sync or perform a manual backup in GoodNotes App Menus.