The following issue happens when you try to preview a PDF in Outlook 2007-2010 only on Windows 64 bit editions. Here’s a rootcause of the issue and its fix:

To enable preview of PDF files in 64-bit OSs, do following:

1. Start Registry Editor (Start - Run - Regedit.exe)
2. Move to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Classes\CLSID\{DC6EFB56-9CFA-464D-8880-44885D7DC193}
3. Change AppID value to {534A1E02-D58F-44f0-B58B-36CBED287C7C}.

Or create a Reg file:

Windows Registry Editor Version 5.00 

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Classes\CLSID\{DC6EFB56-9CFA-464D-8880-44885D7DC193}] "AppID"="{534A1E02-D58F-44f0-B58B-36CBED287C7C}"