That's really strange, they are just .pdf files...
Sounds like your browser isn't supporting them for whatever reason, you could always uninstall/reinstall Chrome/Adobe if viewing them in browser is your preferred choice.
OR
You could always RIGHT CLICK the blue little "Digital Study Guide" option and select
"Save Target AS..." and save the .pdf to your desktop and open it out of browser,
it really is just personal preference when it comes to that.
I save mine to a flash drive so I can study them on the go, as staring at my phone screen for
hours on end begins to put a strain on my eyes and neck.
Anyways, hope this helps.