Luna Display has added support for using an iPad with a Windows PC. The company originally offered the functionality for Macs and iPads but Apple subsequently 'sherlocked' the feature in macOS Catalina with its Sidecar functionality.
https://www.notebookcheck.net/After-getting-sherlocked-by-Apple-Luna-Display-now-allows-you-to-use-iPad-as-a-second-display-for-Windows.572281.0.html