When you take out a subscription the files are stored within AngelNav's private storage. That means you can open them within the app (whether you are online or not), but they can’t be accessed by other software like the Files app (this is a stipulation of the licence from Imray, and also from the UKHO for their charts).
However, if you can source charts from elsewhere (such as the NOAA in the USA) these can be downloaded and treated like any regular file (that is they can be copied, moved, shared etc).
Also, if you have more than one device using the same Apple ID (or Family Sharing) the subscription charts are available on up to six devices.