fill form

What techniques available to fill a PDF form automatically using external data and save them. I have to use data from a database to fill a template PDF and save a copy of it on disk with that data. Language and platform is not issue but it would be good if it can run on windows and Linux.
I have a dynamic XFA Form that I can fill out manually using Adobe Acrobat on my computer. Using iTextSharp I can read what the XFA XML data is and see the structure of the data. But how can I fill out the form programmatically?
Tags: fdfxfdfformsfill formiText 7
This example was written in answer to the question How to save an .xfdf file as a .pdf file?
I assume I need to flatten a XFA form in order to display properly on the UI of an application that uses Nuance's CSDK. When I process it now I get a generic message "Please Wait.... If this message is not eventually replaced."
My code to fill out an XFA form throws no exception and everything seems to be OK, but if I open filled pdf, Adobe Reader says something like this: This document enabled extended features. This document was changed since it was created and using extended features isn't possible anymore.
I have a PDF with text form fields at are layered one on top of the other. When I fill the fields via iText and flatten the form, the form field that I had created on top of the other form field is now on the bottom.
I've tried so many different ways, but I can't get the check box to be checked!