How can I get PDF File Type icon for particular document library in sharepoint 2007

To get the DocumentType Icons in the Grid View, you can go to the Views and Check the File Type Check box as suggested by Chirag.

But in case if you want to get the PDF File Type Icon you need to follow the link or the steps mentioned below

http://support.microsoft.com/kb/837849

Steps

1.    Download an appropriate icon for pdf document types. The PDF Icon is available at the following link

http://www.adobe.com/misc/linking.html#pdficon

2.   Save the .gif file to TEMPLATE\IMAGES directory. (DEFAULT: C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\template\images)

3.   Open the file docicon.xml. (DEFAULT: C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\template\xml\docicon.xml)

4.   Add a new Mapping element to the ByExtension element.
<Mapping Key="pdf" Value="pdficon_small.gif" />
NOTE: Be sure to change pdficon_small.gif to whatever image you downloaded to represent pdf documents.

5.    Save the edited docicon.xml file

6.    Restart IIS (iisreset /noforce).

You should now see the PDF icons for all PDF documents in the portal, including search results.