a pdf online that could be filled?
the only thing i understand from your statement is that there is apps that let you write inside some fields of some pdfs
this was done with adobe reader, do you have adobe reader installed?
afik, okular and foxit can't do this, maybe pdfedit(not sure if it exist on kde4)
about writing inside a website, usually those are html forms that later can be exported by you or the same website as pdf