ORIGINAL URL: https://helpx.adobe.com/acrobat/11/using/display-pdf-browser-acrobat-xi.html
Most Built-in PDFs plug-ins in web browsers are just "Viewers". PDF viewers are unable to submit PDF forms and lack the compatibility with Adobe's proprietary software such as LiveCycle XFA or Extended Reader Usage Rights. End-users will need to manually disable the default browser's default PDF viewer and enable Adobe Reader in order to unlock the full potential of PDF forms. Acrobat XI and Reader XI do not include a preference setting to open web-based PDFs. To change the display behavior, follow the instructions below for your default browser, or see the browser documentation on how to control plug-ins or add-ons.