PlayScore 2 for iOS and Android devices can play PDF scores directly and export them as MusicXML.  On iPhones and iPads, just find them in Safari and chose ‘Send to PlayScore 2’.  On Android devices, open the PDF in your browser and export to PlayScore 2.
On both versions you can open a PDF score direct from the app.
To find free sheet music online try some of the sites below.
IMSLP – The International Music Score Library Project
8notes.com – Free Sheet Music & Lessons
Free-scores.com – Free music scores
The Mutopia Project – Open source Contemporary and Classical instrumental music
MuseScore – Download and print free sheet music for piano, guitar, flute and more