You need to change the registered file type for PDF files in your Folder Options. Here's how to do it:
1) From the Start Menu, go to "Settings", then to "Control Panel" and then to "Folder Options".
2) Click on the "File Types" tab.
3) Scroll down to "Portable Document Format" and select it.
4) Click on the "Edit" button.
5) In the "Edit File Type" window, click on the "Edit" button.
6) Click on the "Browse" button in the window that pops up.
7) Select the executable file for Adobe Acrobat.
8) Click the "OK" button.
9) Click the "Change Icon" button in the "Edit File Type" window.
10) Choose one of the PDF page icons and click "OK".
11) Click the "Close" button in the "Edit File Type" window.
12) Click the "Close" button in the "Folder Options" window.
This should allow Adobe Acrobat to automatically open any PDF files.
