Ok, so it looks like the plugin is fetching the statistics for this, but it is written to only retrieve file downloads (galleys). It won’t show views of the article landing page (I just learned that now by looking at the code! :)).
So in this case the frontend display is working correctly because when you look at the stats in the backend, there are no hits for that article in the PDF/HTML/Other columns.
That means we are back to the original problem: why are usage events not being recorded when you look at the PDF? The first step is to figure out if the usage event logs are recording an event. In your files_dir, look for the usageStats/usageEventLogs directory. Open up the current day’s file and see if new lines are added when you visit a PDF.
You can also look in the archives directory at old logs, to see if visits to the PDF of that article have been recorded. If they are in the archived logs, but the statistics are not showing up, then it sounds like there is a problem similar to the problem being discussed in this thread.