Note: The ZIP files contain the .mz, .txt, and .mid files, not the .pdf files as they cannot be compressed by much. The ZIP files have been produced with PkZip (DOS) 2.50; for zipping with a GUI I recommend ZipCentral: it's the only one I know to display file comments. The .pdf files can be viewed/printed with freely available PDF viewers like GSView or Acrobat Reader. Most have been produced with Acrobat Distiller 4.05 at 300dpi to conserve space; the interested reader can easily produce them at a higher resolution with Ghostscript.

David Webber also provides a free MOZART Viewer so everyone can view and print (and listen to) scores written with MOZART. Under NT5.x (Windows 2000/XP), this viewer -and MOZART itself- can also be used to open the MOZART scores on this site directly: just enter the score's URL in the Open File dialogue, e.g. https://mcmbednarek.tripod.com/nozze/nozze00.mz !

A lot of MIDI files can also be found through the excellent search engine at MIDI Universe.
More free sheet music is available at freesheetmusicguide.