Orux can also read Mapforge and Garmin IMG maps, which is pretty handy.Oruxmaps Can read USGS pdfs and is open source.
It can be free without offline maps. I actually like it so much I paid for the 5-year pro subscription so I can get more map layers (like Nat Geo trails). This has everything I need and more. I use it just about every day.Then Gaia it is! is it a free app?