Someone has asked me to make an ebook of their music book (sheet music). What is the best file type to use? I am hoping that people will be able to see it on as many devices as possible. It is currently formatted as a PDF.
My recommendation is to keep it as a PDF and let people read it by using a PDF app on their tablet.
As long as you have a decent sized tablet (9 inches or more), PDFs can look superb. I don't know if scores have any special requirements for size.
Another option would be to use a fixed layout epub http://wiki.mobileread.com/wiki/Fixed_layout_ePub
The easiest way to do that would be to purchase a one month license to Adobe Indesign, import your pdf into it and do any necessary formatting adjustments (One month licenses are cheap, but the learning curve is fairly steep).
Aha, I have forgotten to ask the key question: do you want to sell it? If yes and you want to do through Amazon, that is a whole different question (your only option would be a fixed layout epub for Kindle).
Even if you say yes, I would still just keep it as a pdf and upload it to smashwords and sell it there (they accept pdf files for selling).
There is something called "the Lilypond music typesetting program". Louisiana music professor Jon Kulp describes how he used it ( and lots of other things) to make an ebook with music examples for his students: