I thought the Adobe Content Viewer was a thing of the past! That was the old testing viewer for DPS. When the new In Design came out in 2017, the Digital Publishing features didn't work with individual articles anymore. It's likely that iOS didn't support that old viewer when updates came out, or Adobe never bothered updating it to be supported on the new iOS. If you are signed up for AEM, you should be able to create your own preflight app for iOS.