Quick fix shortcut
Right click on the .pdf file you're trying to open, click on properties
click change, Another window will open, choose the program to open the file which is adobe acrobat reader by navigating to where that .exe file is located which should be in C:\Program Files\Adobe\Reader ...\Reader \AcroRd32.exe
and click OK
also click OK on properties window
Then try to open file it will open adobe reader by default

In Windows XP:
Go to Control Panel,
Select Folder options, select Tab file types, scroll down to PDF
Click button Change
Click button Browse
Browse to C:\Program Files\Adobe\Reader 10.0\Reader\AcroRd32.exe

In Vista:
Go to Control Panel, Default programs, Set Associations
Scroll down to .pdf
Click on Change Program
Browse to C:\Program Files\Adobe\Reader ...\Reader \AcroRd32.exe