Before answering this question, we must understand what power factor and the harmonic spectrum mean.
The powerfactor is the ratio of real power to the magnitude of apparent power consumed by a load.pf=P/SWhere the apparent power is the vector sum of real and reactive powers consumed by the load. S=VI*
Now we know that the harmonics are integral multiples of the fundamental frequency,Since reactive power is directly proportional to the frequency ( in case of lagging loads)Higher the frequency implies higher the reactive power consumption.
Hence the ratio of reactive power to the real power (or) ratio of reactive power to apparent power is quite high for harmonics , In other words reactive power would dominate the real power (for harmonics).
This CLEARLY means that harmonics have higher reactive power content than real power.
Implying that the increase in net reactive power consumed by the load would be far greater than the increase in net real power consumed by the load.
This obviously means that the power factor would reduce due to Harmonics ( in case of lagging loads)