Is this a service that people need on a regular basis, or is it just a once in a while thing? If it's the latter, that would explain it, people got what they needed and left. If they need this on a regular basis, they probably should find a way to get the data before it is put into a PDF rather than relying on extraction.
This sounds like you'd be better off without a free trial at all. Charge a reasonable fee for unlimited use for a day or two. That way people would feel like they're going to be able to do whatever they need because its unlimited, but you get some renumeration. Just my first thought. i hope you figure something out. Best of luck to you.