On October 15, 2017 Adobe discontinued the support of Adobe Acrobat XI und Adobe Reader XI. Adobe only supports during five years. That means that Adobe no longer provides technical support, including product and/or security updates, for this version of the application.
However the software can still be used!
According to my survey last year Acrobat XI is the most used Acrobat version by my readers. This is also confirmed by the feedback of the attendees of my seminars.