The PBS app is currently supported on iOS 9.0 or later. To see what iOS your device is operating on, open the Settings app on your iPhone or iPad > General > About. You will see a number listed on the right of Version indicating your device's iOS. You can also tap Settings > General > Software Update to see if updates to your iOS are available, however as Apple devices become older, Apple will be unable to provide your device with the latest iOS and your device will show no updates available.
For users with lower versions we regret to inform you that the app will not work on your device.
However, there is a workaround!
Open a browser window on your device and type http://www.pbs.org/ in the address bar. Once you are on the PBS.org website you can do just about everything you can do with the app. You can sign in, create a Watchlist, view your TV schedule, and of course view many of the PBS shows you love.