What op system are you using ?
Windows 7, vista, mac ?
If you double click on a previous adobe pattern does it open ? if not then you can either just download adobe acrobat reader again and it should repair the file association.
If it does then it could be your browser.
I can download the pdfs in explorer, chrome, firefox, opera and safari. Have also downloaded using Dolphin on my mobile, so it looks like the links are ok.
To repair associations in your browser you need to put what browser you are using