Hi nc,
PIP is not an income replacement benefit, many people work full time and, quite properly, also claim DLA/PIP. The only way I could see this causing a problem is if the demands of the job are such that the contents of the claim form could be argued. To give a totally fictitious example; the DWP would certainly look hard at someone who has claimed they are unable to walk and then takes a job as postman!
Nothing on this board constitutes legal advice - always consult a professional about specific problems