I'm having a problem with Skim. Help me!

So, as a graduate student, I read a lot of articles and I've started using Skim to read them so i can take notes electronically. Sometimes, I'm able to highlight the text. However, if the PdF is just a photocopy I made of some pages, I'm unable to highlight and I am forced to use the scroll tool (the hand that grabs) instead of being able to highlight text.

Any ideas?

thanks!