The legal use of PC² is governed by the Eclipse Public License v2.0.
Having said the above, it should be noted that PC² was originally developed, and continues to be developed, by a community of volunteers according to a certain set of community principles and guidelines. One of those principles centers around the concept that we didn’t develop it to make money, but rather to provide useful tools for people who want to run programming contests. We have donated it to the open-source community in that same spirit. As a result, we would take offense if someone were to take our free community project and attempt to make a profit from it.
Note also that the license under which PC² is distributed requires (Section 3.1a) that any modifications which are made to the program must be distributed in source code form. In other words, you can modify it however you like, but if you do then you must freely distribute your modifications.