Thank you for trying to help. We have compared security mode settings and their Macs say that commenting is allowed, while mine says it's not allowed. I'm assuming this is why I can't highlight and they can. But I dont understand why they're different? The files this happens to are PDFs that our college classes assign as reading- so we all go to the same place to access these PDFs and save them, so the security modes shouldn't be different, but they are. Any idea why? Are Macs just enabled to do this for whatever reason and PCs aren't?
Where did you see this information, exactly? Did you look under File - Properties? If so, those are not the app settings, but the file's properties. If that's the case it means the copies you have are not the same after all.
To compare the application settings you need to go to Edit - Preferences.
There's no difference in the ability to add comments to PDF files between the Mac and PC versions of Reader.