PDU stands for "Professional Development Unit." PDUs are credits that are awarded by the Project Management Institute. One PDU credit is given for every structured teaching hour the student attends that is relevant to Project Management.
PDH stands for "Professional Development Hours." PDHs are credits awarded in the general engineering field. The amount of credits required and awarded depends on the particular discipline of engineering. PDHs are gained by continuing professional development in the workplace.
The NCEES (National Council of Examiners for Engineering and Surveying) will provide details of what credits are needed in terms of continuing professional competency. You can check which PDUs and PDHs you will need for the discipline in which you work and what rules apply to your state.