Security Considerations with Enterprise Distribution for iOS
Using Apple enterprise provisioning profiles in your app requires to be extra careful if you don't want it to be available publicly. Un-restricted versions can be downloaded and installed by knowing the app id. The visibility of the download page does not affect whether a build can be downloaded. Additionally, Apple enterprise signed ipa's can be installed by sharing from local storage even when you are using private distribution.
Restrict access to your enterprise signed iOS app: