Just came across the technet article explaining the steps to register the Microsoft Filter Pack, so that Exchange Search can start indexing the text content in different file formats. The filter pack is a pre-requisite for Exchange 2010 installation. After you install the filter pack, the included IFilters are registered with Windows Search. To enable Exchange Search to index Office 2007 file formats, you must register the installed IFilters for Exchange 2010 by modifying the registry. You must perform this step after you have installed Exchange 2010 on the server.
The registry changes can either be made manually or running a script provided by Microsoft. Check the technet article for steps to modify the registry.
The Microsoft Search (Exchange) service has to be restarted after the registry edit.