How to remove user password from pdf on a Mac?

I'm in online school & all of our books are electronic. I recently purchased a Kindle & it worked for my last class, but this course I'm having trouble. I'm trying to figure out how to convert the pdf so I can take my reading with me. When we open the book on a computer, we have to enter a school ID & password. I have looked online for software to remove security restrictions, but the results I'm finding will only remove the "owner" password, not the "user" password. I've found several programs that will remove both on Windows, but I own a Mac. I've looked at Google and YouTube for several hours. Can anyone offer any advice? There are some weeks when our classes have 3-5 chapters of reading, so I would really appreciate the guidance. Thanks!