SharePoint by default does not support and recognize PDFs. Follow this post to display a special icon for PDF files and discover them in SharePoint Search results.
1. Download and install Adobe PDF iFilter 9.
(For more information about how to download PDF iFilter 9, see Adobe PDF iFilter 9 on the Adobe Downloads website.)
2. Download the Adobe PDF file icon.
(For more information, see Use of Adobe icons and web logos on the Adobe website.)
IMPORTANT - Make sure you save the Adobe PDF file icon to the following file location on the SharePoint Server

Drive:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\IMAGES\
3. Add the mapping entry to the docIcon.xml file.
Open the docIcon.xml file in Notepad
IMPORTANT - The docIcon.xml file is located in the, Drive:\Program Files\ Common Files\ Microsoft Shared\ Web Server Extensions \14\ TEMPLATE\XML folder.

4. Type the following entry in the docIcon.xml file.
<Mapping Key="pdf" Value="pdf16.gif" />

5. Click Save and Exit Notepad.
Add the .pdf file type to the SharePoint content index
Browse to Central Administration > Application Management > Manage Service Applications.
Highlight the Search Service Application, and then click Manage on the ribbon.
On the Search Administration page, click File Types.
Click New File Type, type pdf, and then click OK.

6. Click Start - type REGEDIT and press Enter to open the registry.
1) Locate and open the following registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\14.0\Search\Setup\ContentIndexCommon\Filters\Extension
Right-click Extension, click New, and then click Key
Type .pdf, and then press Enter
Click .pdf, right-click (Default), and then click Modify
Type the following GUID in the Value data box {E8978DA6-047F-4E3D-9C78-CDBE46041603}
2) Locate and open the following registry key.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\14.0\Search\Setup\Filters
Right-click Extension, click New, and then click Key
Type .pdf, and then press Enter
Click .pdf and then modify values as below
Default Extension pdf FileTypeBucket 1 MimeTypes application/pdf

7. Exit Registry Editor.
8. Restart the SharePoint Search Service.
9. Restart all the SharePoint servers in the server farm.
10. Test the following scenarios to confirm iFilters setup.
- PDF icon is shown in Type field/column in SharePoint.
- PDF files are shown in search results.
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