With PDF review, the instructions state that you should avoid updating the project after creating the PDF.
Does anyone know if it really is the whole project that shouldn't be touched, or is it safe to work on the topics that haven't been sent for review?
It is important for project structure and the topics included in the PDF. If you completely redo your project structure, RoboHelp might not be able to map the PDF to the topics. You can update topics not part of the pdf. But try to avoid too many changes to the TOC and structure.