Convert .pdf sheet music to .midi files?

Hi! I like working with GarageBand on my Mac a lot. There are ways to convert .pdf sheets of music to .midi files but I don't know how.

Do you know any programs (preferably free, if not that's fine) that will allow me to convert sheet music to sound files? .pdf to .midi?

thank you in advance!
Update: It is possible and there are programs out there,
http://discussions.apple.com/thread.jspa?threadID=840250

has a few but not very good.
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