Use form data to populate another PDF

I'm sending out PDF forms that ask for some basic info (name, email, phone, etc.) as well as some more detailed info. Once I receive these forms back, each respondent will then be sent a follow-up letter. I would like to use the form data they fill out (name, address, etc.) to create those follow-up letters. So, how do I create a PDF whose fields are populated based on a different PDF form's data?

Thanks!

Kris