One of the nice features in elcomCMS is it's ability to automatically insert Google Analytics tracking into documents.
Here's how to set it up:
1. Enable the Google Analytics _gaq.push in Global Site Settings
2. When adding/inserting a document into an article, tick the 'Enable google Tracking' checkbox (if it isn't already ticked)
3. Downloads will appear as Pageviews in your Google Analytics reports
Each time a file is downloaded it is logged as a 'page view' in analytics.
You can filter the list of pages (eg using a part of the download filename):
4. You can also use widgets in your dashboards to show top downloads
Use a filer such as 'only show' and 'PDF' to highlight just the PDF downloads.
Here's how the widget will look in a Google Analytics dashboard: