I realize this thread is OLD, but I just ran into it when searching for text file searching today, and then, when I chased down a couple of suggestions offered here (back in 2010), I ran into the best answer I have seen so far!
The app 'InFileSearch' looks in TXT, DOC, PDFs, and maybe more for a matching string. It is very fast, allows you to open a discovered file while it continues the search, and has a very good interface! I haven't had a chance yet to see if it supports wildcard characters (? and *, for example, or better yet, regex), but I am already satisfied!
I cannot believe that a Google OS (for the Android) neglected to offer the phone equivalent of GDS, sigh!