Just an update. I tried playing around with certain settings and here's what I found:
I updated Evernote to the beta version and switched my chromebook to developer mode.
I realized that any mouse-clicks on the PDF attachments have no effect however when I use the touchscreen (click on the three dots), I am able to open up the usual menu and most but not all menu options are working:
Annotate - works!
View - works!
Copy - works!
Edit - error: Could not find any Windows or native application to open this file
Edit a copy - error: Could not find any Windows or native application to open this file
Remove - works!
Download - works!
I'm not sure if changing to beta and/or developer mode changed things but it certainly seems like the program is not optimized for chromebooks. At least things are semi-functional and usable for me but not ideal... I hope this helps.