Ahhhh - to modify is one thing. Read the fdl for pdf article somewhere in this site. You create a "pdf form" and then supply the data in a separate file. The data from the form description file populates matching fields in the pdf. The only problem is that you have static pdf's where you can really only replace designated areas. You can't make a one page form extend into more pages.
Same thing for rtf...
Now, creating the entire multipage form dynamically gives you total control. Add header, footer, auto page numbers etc and it's not that difficult. Write a function to handle common portions. You can do the same with rtf or pdf - but I found dynamically creating complex rtf was reeeeeeealy tedious.
Anyway, you may want to install the full Adobe Acrobat product and experiment with pdf forms. Btw, you can not only populate these but also post entered data, add java edits etc.
Chris