Subs just gets you extra characters slots and the ability to share content you've bought with other people in your campaign. The content you buy is not tied to a sub. There is an app (both on iOS and Android) that lets you look at content offline. It is currently in beta and they are adding features. But the 'ebook' part of the app seems to be working just fine.
Thanks @lkj and @darjr! Already planning on picking up the Essentials box, and this was what I was whining about in that other thread that WOTC should be doing- extra freebie adventures to support new DM's. Kudos to them.