What is IFilters in SharePoint? Well, as per Microsoft: "The IFilter interface scans documents for text and properties (also called attributes). It extracts chunks of text from these documents, filtering out embedded formatting and retaining information about the position of the text."
Out of the box SharePoint will index many types of content. This includes a lot of popular file formats (.ppt, .doc, .xls, etc). But for some special file types like PDF, TIFF we got to install IFilters in order to index those file types.
You can now index the following files with free ifilter pack: .docx, .docm, .pptx, .pptm, .xlsx, .xlsm, .xlsb, .zip, .one, .vdx, .vsd, .vss, .vst, .vdx, .vsx, and .vtx
You can download the PDF IFilter from: http://www.adobe.com/support/downloads/detail.jsp?ftpID=4025. Follow the KB: http://support.microsoft.com/kb/2293357 to configure it.
Here is the excellent site with list of FREE IFilters: http://www.ifilter.org/Links.htm
Step by step guide on configuring IFilters:
Configuring iFilter for PDF Search in SharePoint 2010 - Step by Step
For MOSS 2007:
Here is the related article on setting up the Icons for custom file types: http://salaudeen.blogspot.com/2013/03/how-to-add-custom-file-type-icons-in-sharepoint.html
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