If you use Adobe Acrobat, Outlook has a Send as PDF button on the toolbar. To remove the button, you need to disable the COM addin.

Go to:
HKEY_LOCAL_MACHINE\Software\Microsoft\Office\ Outlook\Addins\ PDFMOutlook.PDFMOutlook

and change the LoadBehavior value from 3 to 2. Restart Outlook. The PDF functions will still be available in Word and the other Office programs.

(All Outlook add-ins load from either this key or the matching key in the Current User hive.)