I have a sheet music website where I upload the music I compose. However, I want to expand it by adding a directory of popular sheet music. Is it illegal if I simply link to the .pdf files of other sites? Should I contact the webmaster? Maybe I could simply link to the page where the .pdf is? Thanks in advance.
I think that unless those sites are expressively forbidding this, there's no way for it to be illegal - that would deny the very web concept.
It depends on whether the site you are linking to has the right to display this material. Let's say that you link to another musician's website where he has posted his own sheet music. That would be legal. If you linked to sheet music on a sheet music site, where they don't have the right to display it, then that could be illegal (it is in the US and other countries).