You need a pdf editor to edit pdf files now a viewer. I don't know of
any free pdf editors.

Pdfedit from Sourceforge works fine. You can also use some word processors, such
as Kword or even OpenOffice.

I use Ubuntu 8.10 and Document Viewer 2.24.1. It's possible to save PDF
form after typed the form. But, sometime, I did not get right page(s)
format.

Then, I usually use "PRINT to PDF file". Please use "print to PDF" and
don't forget to setup the page first, if needed.