Apps using Android's Storage Access Framework (SAF) will transparently adopt iCloud Documents in iOS.
|Android SAF Interface||iOS iCloud Documents|
iOS requires your developer account to be configured to allow an app to use iCloud. You need to create a container for an app then add the container to the AppID. Consult the Apple Developer website if you are unfamiliar with those terms.
You must have created an AppID for your app.
If must regenerate and download any existing provisioning profiles for the iCloud Documents option to be enabled.