Adobe Reader could not open 'file.pdf' because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded).

I had the noViewRenderer set in my action but that wasn’t enough. Place the snippet below in your controller action:

$this->_helper->layout->disableLayout();
self::getFrontController()->setParam("noViewRenderer", true);