In most cases, sharing data between apps is most safely and conveniently done through content providers.
You can look here for help:
http://developer.android.com/guide/topics/providers/content-providers.html
Or here:
https://developer.android.com/training/sharing/index.html
With a provider the file remains private to one of the apps, it just provides a way for the other app to access the data. Thus any 3rd party app cannot access the file - they may access the provider instead but that is much less discoverable any you can implement some simple access control if needed.