If your instructor has adopted Open Educational Resources (OER) for your course, then the instructor will provide a link to the free, online resources within Canvas. OER are free (and legal) because creators have applied a Creative Commons license to them that grants various forms of free and open use.
See a list of courses and instructors who have adopted OER. This page also includes faculty who have adopted no cost options including library ebooks. Note: Many library ebooks only allow for a certain number of users to access them at a time.
If you are in the possession of a "free" online PDF or other copy of copyrighted material, beware that you are violating copyright and that resource is not legal.