The DjVu format is a heavily compressed composite format so a regular (Agent Ransack) binary data search won't work. However, there is a DjVu IFilter available from Caminova which can be used by FileLocator Pro to read DjVu files. The Caminova IFilter is available here:
However, that won't work for multi-file DJVU files (e.g. using djbz files). Instead if you have a program installed that can convert a DJVU file to text you can setup a custom extension instead. Here's an example created by FileLocator Pro customer using DjVuLibre:
Save this file to your plugin_cfg folder as djvu.xml
<setup>"C:\DjVuLibre\djvutxt.exe" "$(InputFile)" "$(OutputFile)"</setup>
Then check that it appears in the Configuration->Extensions Settings and confirm that 'Use Indexing Service Filter' is unchecked. That should be it.