Search textstring in multiple filetypes

Hi,

I'm looking for a Search program that can find a textstring in multiple files of different types in one go.

O3find.exe can find a textstring in multiple txt, xml, doc, xls, ppt, odt, ods, odg, odp files,
but unfortunately NOT in: docx, xlsx, pdf.
(http://web.tiscali.it/fanelia/sw/o3find/)

Does anyone know about a program that can handle the complete list, or can O3find be modified ?

Thanks