Many websites offer free sheet music for guitars including 8notes, Musescore, IMSLP/Petrucci Music Library and Free-Scores.com. 8notes has plenty of free guitar sheet music available for download. The site also offers sheet music for other instruments, including piano, violin, cello, percussion and more.
MuseScore is primarily a software that lets users create, play and print sheet music, but it also has a "Sheet Music" section that allows the free download of sheet music in PDF and music XML formats.
Free-Scores.com is a French website that users can translate into English using Google Translate. It is a good source of classical music pieces. Use the search box to locate the sheet music you need.
IMSLP holds plenty of public domain sheet music. Use the search box to find sheet music, then click on the results to download the PDF file.