In addition to having the PDF file in both letter- and A4-sized documents, each piece comes with a downloadable MIDI file.
While I'm at it, here are a few more free sheet music sites:
Project Gutenberg has its own sheet music "department," and the Choral Public Domain Library has a tremendous collection of sheet music. They have been collecting music since 1998, and have 8324 scores to date. The Werner Icking Music Archive has other useful (and eclectic) pieces of information and links to various articles of interest, like a repository of common engraving mistakes.
I find this all very exciting.