Please try these steps and see if this fixes the problem:
1. Open the "Start" menu and click on "Default Programs."
2. Click "Associate a file type or protocol with a program." The window lists the types of files stored on your computer.
3. Select ".pdf" from the list and click the "Change program..." button in the top right portion of the window.
4. Select the program you want to open PDFs. Windows will display some recommendations. If you don't see Adobe Acrobat Reader in the window, click the "Browse..." button and browse to the location of the Acrobat Reader installation on your computer. Select the program and click "Open." Your computer should now open PDFs with Adobe Acrobat Reader.
First of all, thank you so much for your help Ambucias. Yes, in order to see "Adobe Acrobat Reader" in "Default Programs", "Adobe Acrobat Reader" must be first installed in your computer. Please install the "Adobe Acrobat Reader" and post the results.