I am using Offline Maps Android App: Maps.me (link to web page)
App is for free. Offline maps do not need online internet.
When you are conected to WiFi, in this app, you can download offline map of each country (houndreds of megabytes).
In these maps, there are even some popular treks like Bajkal trek, Everest BC trek or Annapurna trek.
You can put bookmarks into maps (like booked hostel, favorite places).
=== Knaper ===