Possible to edit link on portfolio items?

I’m test-driving WP Portfolio. I see where I can edit the slug for portfolio items but I can’t actually specify the URL I want (am trying to link the portfolio item to a PDF file in my media library).

I’ve looked all the through the settings and the closest thing I could find was “slug” and “taxonomy” slug but when I type in a value and hit “save” the field reverts to blank again.

Is there a way to control the URL of portfolio items?