Adobe Acrobat 8 Pro License- Not Valid on Adobe XI PRO

Hello,

I want to set my Adobe Acrobat 8 Pro as my default for opening PDFs.   However, as I am a CC customer with Photoshop and Lightroom, Adobe XI Pro always opens up when I double click on a PDF.  It then asks me for my license -- I type in my serial number-- and it's not valid.

How do I . . .

1.  Make the prompt for Adobe XI Pro go away so my PDFs just open with Adobe Acrobat 8 Professional

or

2. How do I use my serial number with Adobe XI Pro

Running 10.9.4 on an iMac (2.7 GHz Intel Core i5) with 16GB Memory