How can i create a trigger for a PDF download?

I have a PDF application form uploaded to the design studio. It is linked from a landing page and i need to create a trigger to send an alert when someone donwloads the application form. Does anyone know how to create that trigger, it seems to be just for Email links or a webpage?

If i add it as a webpage it gets the red squiggle underneath to say it's not valid, i'm not sure if this will work and is just a bug in Marketo.