When adding an icon to a button labelled 'Download free ebook' one can use an icon to represent either the action (download) or the subject matter (PDF). Which is better? Why? (And is the answer impacted by context or intended user?)
Imagine you have designed a music player and the play button icon represented the format of the song that will play when you click it, i.e. instead of the "play arrow" it had an icon representing the "mp3" format of the file. Would that make sense?
If a button triggers an action then it usually makes much more sense to have the icon reflect the action. The information about the item being acted upon can be indicated elsewhere (in the music player example it would be in the playlist, or a "currently playing" display of some sort).