Eloqua Best Practice Needed for Sending Email Alerts When PDF's Are Viewed From Storage Locker

According to my reading, there is a tracking link associated with each upload in the file storage area, which if used provides a signal as to which subscriber has opened that PDF asset. 

The part that I cannot figure out is how to "listen" for that particular, tracking link, and how that works in a program to initiate subsequent actions such as sending an email. 

I would've thought that it was the listener that did that unfortunately, the listener was limited to scoring, field changes, and other things. 

I could not find a selection on any pick list that offered URL tracking.