David is looking for an app that will turn his iPad into a secondary display. Leo suggests Duet, which was written by former Apple Software engineers. He can make it mirrored or extended and it works on both Mac and Windows. It uses remote access from his computer. This requires a wired connection to the computer, though.
There are several other options out there if David just wants to remotely access his computer over Wi-Fi. TeamViewer is one option. There are many more if David just searches the app store for "remote access Windows."