pdf submission won't advance pages

by g k.  

I have an assignment set to accept pdf submissions. But when I try to grade the pdf, I cant seem to advance beyond the first page of the submitted pdf. The page advance button literally does nothing.  Any ideas?  

I am running Moodle 3.1.2 (Build: 20160912) on a Linux web server using the Adaptable theme--although it doesn't seem to matter what theme I use.