How do I track files (such as PDF, AVI, or WMV) that are downloaded from my site?
From the website:
How do I track files (such as PDF, AVI, or WMV) that are downloaded from my site? – Analytics Help
Google Analytics provides an easy way to track clicks on links that lead to file downloads. Because these links do not lead to a page on your site containing the tracking code, you’ll need to tag the link itself with the _trackPageview() JavaScript if you would like to track these downloads. This piece of JavaScript assigns a pageview to any click on a link – the pageview is attributed to the filename you specify. (Read the article for further details.)