In order to fix the issue, upgrade Adobe Reader to the latest version. After that, go to your browser options, open the Plugins section and enable the utility. If it doesn't work install another PDF viewer.
If you're using Opera, navigate to Opera add-ons page, click the green Add to Opera button and follow the on-screen instructions. For Mozilla Firefox, go to Mozilla Firefox Extensions web page and press Add to Firefox. If you're using Chrome, navigate to Chrome Web Store and add Docs PDF/PowerPoint Viewer to your browser.