Steps to replicate:
- Log in and enable media viewer beta feature
- Visit a page with a PDF embedded, like https://meta.wikimedia.org/wiki/Metrics_and_activities_meetings/Quarterly_reviews/Editor_engagement_experiments/2013-04-12#Slides
- Click on the PDF
Nothing happens for me at this point.
If I open the same page in an incognito browser (Chrome or Safari), the File page for the PDF immediately opens.