Searching through txt, pdf, and doc files

agent ransack. ( just google it)

has the advantage that it searches pretty fast on networked drives too.

PowerGREP is another suggestion.

From their website:

PowerGREP is a powerful Windows grep tool. Quickly search through large numbers of files on your PC or network, including text and binary files, compressed archives, MS Word documents, Excel spreadsheets, PDF files, OpenOffice files, etc. Find the information you want with powerful text patterns (regular expressions) specifying the form of what you want, instead of literal text.

It's not free, (it costs $149) but it appears to be the best fit for what you're looking for (except the DJVU one - I haven't found anything that can search through them yet.)

I use WinGrep for this purpose. Free, small, works really good.