The ultimate source. Read the on viola:

The viola’s naturally mellow and melancholic timbre perfectly captures the "pensive feel" of Einaudi’s work. Unlike the violin, which can sound piercing in the high registers of this piece, the viola provides a rich, grounded quality to the arpeggiated figures that define the song's middle section. Where to Find Viola Sheet Music

Experience Ludovico Einaudi Viola Sheet Music [best] Here

The ultimate source. Read the on viola:

The viola’s naturally mellow and melancholic timbre perfectly captures the "pensive feel" of Einaudi’s work. Unlike the violin, which can sound piercing in the high registers of this piece, the viola provides a rich, grounded quality to the arpeggiated figures that define the song's middle section. Where to Find Viola Sheet Music