When you open the Scanner settings, there are two toggles to manage the app's connection with iCloud.
The enabled Use iCloud for Sync toggle saves the scans to the iCloud server in their original format. This syncs the scans between different devices and also allows to load the scans back from the server after the app is reinstalled or updated.
The enabled Share Scans in iCloud Drive option automatically uploads the PDF created from the scan to the Scanner by Readdle folder in the iCloud Drive storage.
Note: If the scan was deleted from Scanner, it would be deleted from both iCloud and iCloud Drive.