Why downloading is pauses?

If you are trying to download an iOS update and you find that the update is paused, this could be due to several things.

If you are not connected to Wi-Fi

Solution: Connect to Wi-Fi here, Settings → Wi-Fi

If your iPhone has less than 50% power:

Solution: Connect a charger

If your iPhone is in Low Power Mode:

Solution: Disable Lower Power Mode here Settings → Battery

You can find more useful information here:

If your iPhone or iPad won't update - Apple Support