I had the same problem. I guess you are using Firefox?
At one time in the past you have picked Adobe Reader as default when it comes to downloading PDF-Docs. Unfortunately, AR doesn't support that option. To change it back type about:config into the adress-bar of the browser. Search for "browser.download.preferred.application/pdf". Underneath it should say "Adobe Acrobat". Reset this option. Next time you download a PDF Firefox will ask you how you want to open the PDF. Pick anything but AR.