The Internet Music Score Library Project is an online library of public-domain sheet music. Most of the available music is in PDF format and can be freely downloaded. The files are uploaded by users, mostly scanned from published sheet music that falls into the public domain. This means mostly compositions that are old enough to be public domain, in published editions that are also old enough to be public domain.
This is a fantastic resource for finding older editions of woodwind solo pieces, chamber music, and orchestral parts.