How to read any office document

Hi!

I'd want to know if there's any class or library to open office documents and then read them.

With the conventional method, creating a StreamReader and so, sometimes strange characters are read, for example if the document contains tables or images.
What I want to do is, for example, open an existing file (.doc, .xls, .pub, .ppt etc., even .pdf) , read the whole document  and then count how many times appears each word.

Is this possible?? How??
I found a library called iTextSharp which allows me to create documents but I'm not sure if I can read them, and if so I don't know how because there's no documentation.

Thank you very much, I hope there's someone who can help me,
                               
                                    Susana