If someone’s already posted this, I’ve missed it:
If the composition and edition are in the public domain, you have a better chance. I’m not an expert, but I get the impression that classical music tends to get re-edited every so often; the actual notes aren’t changed, but the publishers add new commentary, introductory matter, notes on performing specific passages, and finger markings. All those things are copyrighted, so in many cases all you get to download are the notes and staves. For some instruments, that can be a problem except for very advanced players.