As Christophe mentions, articles that have already been viewed are cached locally on the device for offline use. Note that the app will, if necessary, delete the oldest accessed articles to free up space.
If your reader wants to guarantee a collection is available for offline use we have a new feature on iOS that came out on Friday that allows for explicit downloading of a collection and prevents it from being automatically purged. See Digital Publishing Solution Help | What's new in this DPS 2015 release for more information. This feature is currently on iOS, and will be available on Android and Windows in a few weeks.