Hi there!
Express does not offer redaction support (even on the client), but you might want to check out our sister product, PDFTron WebViewer. It offers full client side redaction support and many other features that are not included in Express. Doing everything on the client is cheaper much more scalable as you do not need to setup and maintain a server.
If you still want to do it server side, PDFTron also has a .NET SDK that can do redaction on the server.
If you wanted to combine Express with the .NET SDK I linked above, you could implement your own client side redaction tool (by just extending a rectangle annotation) and then convert that annotation to a redaction on the server. If you choose to go this route, we could provide guidance on the implementation.
I hope this help! Let me know if you have any more questions.
Logan