hawkguy

To add to mummelpuffins comment:

For 5e there is https://open5e.com/, although that is under the OGL.

But for B/X and other OSR-systems there are whole lot more, I think. Basic Fantasy RPG is with it's 4th edition OGL-free:

http://www.basicfantasy.org/downloads.html

I am trying it now for the first time on Android. But everytime I want to go back with the back button of the navbar, the app quits. Is that intended behavior?

Thank you for crossposting.

The adventure itself looks really intriguing and I am going to remember it for when I get around to play Cairn.