Outlook simply uses the File Type Associations in Windows to decide which
program to use when opening a file. Try saving the .pdf file to the
harddrive and then double click on it and the same thing should happen. If
it does then close Word and right click on the file and select Open With
and then select Choose Program (If using WinXP). Then select Adobe Acrobat
and check the box the says, "Always use the selected program to open this
kind of file". Then test again and it should work fine. I hope this
information is helpful.
Robert Findlay
Partner Technical Lead -- Outlook/Office Setup
Microsoft Technical Support for Platforms and Business Applications