I often search the web for sheet music for myself or for the people I teach guitar to. There's a lot of free sheet music to be found on the web, certainly for classical music, because it often is in the public domain. Those pdf's can be scans of old sheet music and sometimes are of bad quality and hard to use if you want to play from it. So I re-typeset them and I add fingering or tabulature.
Apart from classical music there's also a lot of music for children and folk in the public domain. In this case the situation often is a lot better. Folk musicians makek use of ABC-notation. It's not only a way to name notes but also a program to write sheet music that turns it into pdf's. It's rather limited in features for high quality sheet music though. That's why I use lilypond.. It is similar in 'coding' the sheet music, but it is a lot more advanced.