This may be old news, but this might help:
To manually enable or disable the PDF File Preview integration within Microsoft Outlook, please follow the instructions below:
Within Microsoft Outlook 2010, go to "File" menu and click "Options".
Select the "Trust Center" link.
Click on the "Trust Center Settings..." button.
Select the "Attachment Handling" menu.
Click on the "Attachment and Document Previewers..." button.
Locate the "" option from the "File Previewing Options" list.
To enable the PDF File Preview option, place a check in the "PDF Architect Preview Handler" checkbox.
Note: To disable the PDF File Preview option, remove the check from the "PDF Architect Preview Handler" checkbox.
Click "OK" to apply the changes.