Unable to take screenshots of PDF files opened in Chrome

Unable to take screenshots of PDF files opened in Chrome

Hello, 

I'm a new user of Nimbus Capture. I usually open my PDF files on Chrome. Sometimes I have to take some screenshots of certain parts of the file do add to a .doc document. 
But, I can't take captures using nimbus. 
I open the file on Chrome, select capture tool and nothing happens. Is there anything I should do? Thank you!