Usually a PDU can be mounted in any orientation. However, when mounting a PDU with circuit breakers, you must obey these rules:
Circuit breakers CANNOT face down. For example, do not horizontally mount a Zero U PDU with circuit breakers on the ceiling.
If a rack is subject to shock in environments such as boats or airplanes, the PDU CANNOT be mounted upside down. If installed upside down, shock stress reduces the trip point by 10%.
Note: If normally the line cord is down, upside down means the line cord is up.