If you install Microsoft's Windows Desktop Search (free), you can point it to your PDF folder. I'm guessing you already knew this, and you need something more customized.
As far as I know, there are free tools on the Web for extracting text from PDF files using VBA. And presumably you can use the Word object model to convert DOC files to plain text. Once converted, VBA can search the text.
If that doesn't pan out, CodeProject has a free library called Epocalipse which employs IFilters (assuming you installed the Microsoft Filter Pack and/or Windows Desktop Search which installs the filters) but unfortunately was written for Vb.Net, not for VBA. You can probably get it to work in VBA, though, because VB.Net has provisions for converting a VB.Net DLL to a COM DLL.
