To get MusicXML: in MuseScore, open your score and choose File → Export → MusicXML (.mxl). Many pieces on musescore.com and IMSLP can also be downloaded as MusicXML. PDF and photos are not supported yet.
The keyboard has one octave: you only pick the note name, and the app works out the octave. A black key counts for both spellings, so C♯ and D♭ are the same key.