Why can't I and how can I read files on my Macbook?

In school, we use a program called Blackboard. I have an online class that requires me to download different files and powerpoints for different assignments. When I click it to try and read my assignment for the week, all that pops up are random letters and symbols. How can I change this? I have a feeling I’d need to convert the file but have no idea how. Also, I noticed that this happens to a lot of documents which have been typed on Word. What should I do? Thank you!