Hello,
I was having this same problem and sent Dennis's solution to our IT guy. His response, which worked quickly and easily, is below:
It seems like I recall updating your Acrobat Reader the last time I worked on your computer. When I did that, I may have set Reader as your default pdf viewer. Based on what youre describing, this may be whats causing the problem. Designer may require Acrobat to be your pdf viewer and NOT Reader in order to give you the option you mention.
Lets try this first:
1. Double-click My Computer or select it from the Start menu.
2. Select Tools > Folder Options, and then select the File Types tab.
3. Locate PDF extension from the list and click Change.
4. Select the version of Acrobat or Adobe Reader you would like to use as the default viewer and click OK.
5. Click OK to close the Folder Options window.
Good luck to all,
Kim