There are a number of sites which have sheet music in PDF format available for download - for FREE! Here are some of them:

• http://www.imslp.org  I pull a lot of music from this site for Pinellas Youth Symphony and for Serenade Strings
• https://imslp.org/wiki/User:Geiusboy98  (list of important composers and their PD dates for Canada)
• http://www.violinandviola.co.uk 
• http://freesheetmusic.net 
• http://www.violinonline.com 
• http://icking-music-archive.org 
• http://www.8notes.com 
• http://www.music-scores.com 
• http://www.free-scores.com 
• http://www.sheetmusicarchive.net 
• https://www.marineband.marines.mil/Audio-Resources 
• https://bandmusicpdf.org 
• https://www.musicaneo.com/sheetmusic/ 
• https://www.brassexcerpts.com 
• http://en.scorser.com/D/Sheet+music.html 
• https://musescore.com/sheetmusic  (note the a .com URL)

Classical Archives sadly no longer is free. It has 14-day free trial, but monthly/yearly fee, and is in various formats. Back when it was totally free, I used to download from their large selection of MIDI files to pull into music software as a starting point.

• https://www.classicalarchives.com/