I guess you can also try going to the web activity report, export the report, sort the visited web page or do a find on a .PDF so you can see who viewed the PDF files in one report. If you use google analytics, you can go to the contents part of the analytics and do a search on .PDF for a specific time frame and it'll show you the pageview, unique page views, avg. time on page and etc. I'm not sure if that's what you're looking for, but that's what I do on my end.