A couple of ideas pop into my mind right of the bat (wihtout knowing too much of what you're trying to accomplish).
You could create the dropdowns for each option you might have, but that can get unweildy in a hurry depending on how many options you have in each dropdown. e.g., if you have 4 choices in the first dropdown, 4 choices in the second dropdown for each option in the first, then each of those choices gets a different third dropdown, you're talking about 21 different dropdown menus to manage.
You could fill the dropdown based on choices in each dropdown menu. That takes some of the pressure off of the PDF reader to handle what you gave it, but the trade off is the more difficult programming that's required. eg. based on the first choice, you push items onto an array, and that array becomes the choices in the dropdown.