I have the same issue, however I could open the PDF files untill December last year (2016).
It must have something to do with a upgrade in either adobe or eccouncil or iOS
Beginning last year, Apple began pushing App developers to support only the most recent and most secure SSL encryption standards. Supporting these standards means that communication with some older SSL servers would not work (as you describe above). Initially, Apple stated that the deadline for this support would be the end of 2016 and that apps would not be accepted for submission without ATS support. We made these changes anticipating the requirement from Apple and, most importantly, to protect our users. Apple has since extended the deadline, though it isn't clear for how long. Regardless of the deadline, we believe that this change is required to protect communications between our application and the services it connects to, upholding the security promise of Adobe Acrobat Reader. The Adobe Acrobat Reader iOS App was updated in December to only support secure communications. The server administrator will have to upgrade the server to secure these communications, enabling Acrobat Reader to connect.
More information on the changes required for the DRM service is here: