In this Google Tag Manager tutorial, the good guys at Amazee Metrics will walk you through the process of setting up tracking for downloads from your website.
The setup below will allow you to keep track of the number of times users download different types of files from your website.
Every time a user clicks on a link that contains the file extensions you define (for example, .pdf) an event will be recorded that is labeled with the type of the file and the URL of the file.
You then can easily see in Google Analytics how many pdfs, for example, were downloaded from your site, and which ones.
www.amazeemetrics.com/en/blog/google-tag-manager-tutorial-part-2-tracking-downloads-in-gtm-version-2