Are you using a jQuery onHandler or clickHandler extension to fire the tag on click? If so, you can add an extra data source as part of the configuration that uses jQuery to pull the text of the clicked link and pass it along, utilizing the following code (assuming that the jQuery selector is the anchor link to the pdf):
From there, you would then just map that data source to the u4 value in the DoubleClick tag configuration mapping toolbox.
Following this answer is an image of what that would look like, where I created a new data sourced called pdf_text and am using jQuery in the JS field to achieve the above. pdf_text would then be mapped to u4 in DoubleClick Floodlight.