Us UNIX folk have been using grep since the flood, but there's hope for you poor Windows folk.
Grep for Windows
Get ready for a whole new free finding experience!
Grep is all well and good (Windows has find but does not have the regular expressions ) but both pdf and doc/docx are binary formats so unless you hit lucky you are unlikely to find consecutive characters of a word grouped together for it to find.
grep is only really useful on text files or at least files where you can rely on a pattern match which the two suggested formats dont.