EDIT: A free copy of the rulebook is said to be available from the creator's website, and it's the same copy as described below, but the one described below might not always be on sale free of charge. A .pdf copy of the Burning Wheel rulebook appears to be available legitimately free of charge at drivethrurpg.com. It's a gaming rulebooks, novels, and media store that normally charges money for products, but charges nothing for Burning Wheel. The rulebook comes bundled with some other stuff, including charsheets, in a zipfile, and you do have to register with an email address and go through a checkout process requiring no additional information in order to download the zipfile. Since I obtained it free of charge, maybe the Rule20 Terms of Service permit me just to put it in my Google Drive and provide players with a download link. I'll ask around, but if anyone knows for a fact whether that's allowed, please let me know.