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If you are using any of the Microsoft Search tools like MOSS 2007, Desktop Search, Vista Search, SQL Server FreeText Search or Indexing Server you will need a piece of software called IFilter. IFilter is a set of DLLs designed for each file format that you want to be indexed and searched.
Microsoft ships for their file formats in Office (Word, Excel, PowerPoint) when you install the Office Suite in the machine. Out of box, the search tools include for IFilters for Text, HTMLs, etc. But if you need to index additional file formats most commonly for PDF (Adobe Acrobat) you will need to download IFilters for them and install. The installation will not give you any User-Interface Application or will you need to configure anything. Just install the IFilter package and reboot your machine or indexing service. That's it.
The sources for getting IFilters are many, I have listed few of them below: