Do you have Adobe Acrobat Reader installed? That's a .pdf file, and in order to view it you need either Acrobat Reader or another program that can open .pdf files. If you don't have it, you can download it free from
http://www.adobe.com/products/acrobat/readstep2.html
If you've already got Acrobat Reader installed, you've got a problem with your file extension associations. To fix it, open Windows Explorer and select Tools then Folder Options. Click the File Types tab and scroll down to PDF. Click on it to select it, then click the Change button. In the new window that opens, scroll down to Acrobat Reader and click on it to select it, then click OK. Back in the Folder Options window, click OK. After that, you should be able to open the file.