Your pieces. Edit the title and composer (shown atop the score heatmap), or add a new piece.
Need a score file? Browse musescore.com ↗ — filter to Solo piano and free to download, then save the piece as MusicXML. For most classical music that filter leaves plenty, so you shouldn't need their paid subscription. A PDF or a scan won't work: pialog needs the notes as data, not a picture of them.
Works best with solo piano scores. Repeat signs, 1st/2nd endings and pickup bars are all handled. Extra parts — a vocal line, a second instrument — are included in the music pialog listens for, so it will expect notes you never play. D.C. / D.S. / Coda jumps aren't supported yet; use a version with the repeats written out.