The Social Sharing plugin allow to share a file inside the app:

https://github.com/EddyVerbruggen/SocialSharing-PhoneGap-Plugin

The contents page should be modified for adding a new "Share" option for contents downloaded or, since this plugin uses also "Intents" in Android we can replace the current option for opening documents, so wen a document is opened by default you are doing a "Share" option and you can choose how to open the file (with which app)

Plugin:

https://ionic.io/docs/supported-plugins/social-sharing