Open your 'My documents' folder and click on Tools/folder options.
Now click on the File types tab.
Scroll down the list and see if PDF is there. If it is, click on it and then click the 'change' button and select Adobe Acrobat. Put a tick in the box marked 'always use the selected program to open this kind of file'.
If PDF isn't listed in the file types list you'l need to add it. Click on the 'New' button underneath the list and type PDF in the extension box, and click on 'advanced'. You should see 'Adobe Acrobat document' appear in the box marked 'Associated file type'. If you don't you'll need to scroll down the list and select it.