Thousands of iOS apps are released every month, yet only a couple really turn out to be any use. This is the case for Duet Display - an app developed by ex-Apple engineers that transforms your iPad (or iPhone) into a secondary display for your PC or Mac.
Just launch the Duet Display app on your device, launch the helper on your computer, then connect your device to your computer with a lightning cable. Your iPad will now serve as a second display for your computer.
The downside to this method is that the Duet Display app costs $15.99 - however if you know you'll be using it a lot, it's a more than fair price.