Yes. Exploria is a free service.
Yes. Exploria has been built to work on the most popular versions of Apple's iPhones and iPads as well as phones and tablets which run with Android.
Yes. When you search for stories, the playlist or list of stories, and their text is downloaded to your phone so you can view or listen when you don't have reception. When you're leaving a town, make sure you have your playlist ready so you can listen if you are offline.