Not sure if Viewranger maps can be used in other apps, but it can certainly be used on more than one device. I bought a DVD from Viewranger years ago, to have maps on my Nokia N95. I put all of them on a card in the phone, which worked after a fashion; utterly crap phone, needed a BT receiver to function properly.
When I finally changed to an iPhone, and Viewranger finally produced an app, I side-loaded all the maps, 1:50k, onto the phone, and I’ve just carried on using the same set of maps, with extra 1:25k tiles, ever since, currently on my iP6+, my old iP4, and the previous iP5, which I haven’t got around to selling, as well as on my iPad.
As the maps are on a DVD, in folders, it may be possible to use them in another app, I accessed them via a web URL.