With Outlook 2007 you don't have to open Word or Excel to view an attachment in a mail. But you can use the built-in previewers. Foxit PDF preview handler released a pdf previewer which works with Outlook 2007 running on Windows Vista. Download Foxit
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Read the complete post at http://blogs.technet.com/aralves/archive/2007/03/01/outlook-2007-pdf-previewer.aspx