As mentioned in the Compatibility section on the app webpage, the app in question requires iOS 8.0 or later to run. You can only download the legacy version of an iOS app if the app developer continues to support it. And in case they do so, you should be able to download it using the App Store app on your iOS device.
If you are not getting the option to download the app when accessing it via the App Store app on your iOS device, the developer is no longer supporting the version of iOS running on your device. And there would be no way to download the app from anywhere else on the Internet.
In general app developers update their app with a major focus on supporting latest and recent version of iOS while dropping support for older versions.
If you have a serious/justifiable reason to run the app on older version of iOS, you can try contacting the developer via the Developer Website mentioned on the app webpage. (http://www.av380.cn/ in this case)