As of clarification via comment, you really don’t need a additional version, subdomain with the same content. What you need is to get the post content, nothing more. The styling and such should happen in your app, not by pulling it from the site. At least that’s how I feel about it.
Anyhow, for this you can use WP REST API. It is partly integrated into core and will be integrated completely, for now you can extend on the already implemented functionality by using the plugin version of the API. Below I link you some resources. There is a stable, but deprecated, version 1.2.5 and a version 2.0-beta13.1 of the plugin. Personally I would definitely go with the version 2 beta, because it is well on its way. But you have to read up on it yourself, so you can make a educated decision on your own.
- Make WordPress Core: WP REST API
- GitHub: WP REST API
- WP REST API V1 @ WordPress Plugin Repository
- WP REST API V1 Documentation
- WP REST API V2 @ WordPress Plugin Repository
- WP REST API V2 Documentation
Edit:
Ok, another suggestion, but as said in the comment, I am not convinced it is the best approach.
You could duplicate the content in a similar way as MultilingualPress does it. It works with single sites of a multisite setup. Although it is a translation plugin, there isn’t actually a need to translate. Besides this just a proposal, which you can use for orientation.