I'm looking for a plugin to extend the search into the content of attachments (typically text file like doc or Pdf).
This topic in the WordPress support forums addresses this question, and suggests Google custom search, which should in theory search PDFs and DOCs and would be easy to set up and try, or the Search Everything plugin, which might be what Jeff said doesn't work in his comment above.
There's also the Sphider Wordpress plugin, an search plugin that claims to support indexing PDF and DOC files. It might be more difficult to set up, but it's the best option I can find.