Add dynamic field to Gravity Forms on Wordpress

We have a WordPress site that I manage now after our developer left. There is a page where if users select white papers, and clicks "Download PDF" they are prompted with a Gravity Form to enter basic information and then are allowed to download the PDF. The problem is we want to track what PDF is being downloaded. If the user clicks on the title of the PDF, they go straight to a special page and can still download the PDF. If they do it from that point, it is as simple as capturing the URL via a merge field in Gravity Forms. But if the user selects download PDF from the filter page, we do not know what PDF is being downloaded. We would need to introduce a dynamic field or custom merge field that is hidden and sent along with all other form fields. I have a lack of knowledge of Gravity Forms and PHP so I do not know how to programatically do this. You can test this for yourself. Here is the URL: [login to view URL] If you click "White Papers" at the top left for a filter, these are the documents that prompt for basic contact information. If you immediately click "Download PDF" at this point, we would like to capture which PDF you are downloading, we can't do that with the URL merge tag. Only if you first click the title and then "Download PDF" can we use the URL merge tag. Any help much appreciated. Developer should be versed in Javascript, Wordpress and the Gravity Forms plugin. Should be very simple if you have these skills.