Free Piano Sheet Music is a phrase used to describe music that has been downloaded freely from the internet and then printed out for playing at the piano. Technically therefore, the phrase is being misused and could be more accurately known as "printed midi files". Essentially a computer user will browse the internet using a search engine such as Google and will search for "Free Piano Sheet Music".
This will result in a long list of websites being presented to the user, all apparently to do with Free Piano Sheet Music.
But how you can get free piano sheet music from a website when by its very definition, sheet music must be on sheets, or paper and this isn't possible to do through a computer screen.
But of course, the midi file is indeed then printed out which leaves you with a certain type of free piano sheet music - it is sheet music that YOU paid for the paper for, YOU paid for the ink and YOU paid for the internet connection in the first place.
So I hope we can all stop using the phrases free piano sheet music and come with a more accurate name. My own nominations include Free Midi Files for Printing and Free Virtual Sheet Music - Just add Ink!