How can I connect my Windows laptop's keyboard to my iPad? I have a Windows laptop and an iPad Air 3. I want a physical keyboard for my iPad but I don't want to carry around another Bluetooth keyboard. 

Is there a way that I can use my laptop as an external keyboard for my iPad?
You may want to have a look at an App called AirType.  According to the developer, it works on Mac, Windows and Linux machines and the requirements are iOS 8 and above as well as both devices being on the same network.