In iPadOS, files that are stored in SharePoint can be imported using the Microsoft OneDrive integration.
First, make sure you have the OneDrive app installed on your iPad.
Then create a new Case File, and once you are inside the Case File, tap on the Plus button underneath the Documents List, tap on Import Documents and navigate to and open OneDrive. Inside OneDrive, select Libraries to open and access your SharePoint Document Libraries.
If you don't see OneDrive, but you have the app installed on the iPad, tap the More Options button (ellipsis button at the top) at the top of the Locations sidebar and choose Edit. This will allow you to turn on One Drive so it will be seen as a location in the sidebar.