GA doesn't track file downloads automatically. Google recommends adding an 'onClick' event to each anchor tag you use for file downloads. This can be a bit tedious. But there is a download tracking script called 'entourage.js' that automatically tracks the links for you. By loading the entourage.js JavaScript code at the start of the page, all document links will have such tracking added automatically so that the downloads appear in the usual 'Content' reports of Google Analytics.

More information and download of Entourage is available on Tech Octave web site.