Searching strings in raw disk image

I am currently writing a tool that should scan a **readonly** raw disk image for a a given pattern. The task is to get the byte offset of the match. I am able to find simple text documents with grep -a -o -b -iE PATTERN IMAGE, but i can not find a way to search through excel, pdfs and word documents. I know that _grep_ is not suitable for this task. Is there an other tool i can use?