iPad 1 can't install a new app I want to install Qi Dian on my iPad 1 but I can't. I've tried to download from my Mac first but this app can't be download by Mac, only iPad device or iPhone. How can I install this on my iPad 1?
The original iPad (iPad 1) can support iOS 3.2 up to iOS 5.1.1 - source here: the iOS Support Matrix.

According to the link you provided for the app, the app you want to install requires iOS 8 or above.

This means that you can not install the app on your iPad, since it requires iOS 8, but your iPad can only run up to iOS 5.1.1.