The PDFreactor service is run on the application server Jetty. It is a requirement for the .NET and PHP wrappers.
Jetty is configured for a development enviroment, and will only listen at localhost at the port used by the .NET and PHP wrappers.
By default Jetty will listen at localhost:9423.
http://jetty.mortbay.org/ for details about Jetty and ways to configure it.