PF or Power Factor is a measure between the "distance" between the voltage waveform and the amperage waveform.
Power factor is created by load, not by the generator.
1PF is typically a resistive load, such as a kettle, this means the amp and voltage waveform are directly ontop of one another.
Between 0.4PF and 0.98 is typically inductive loads, such as motors, fluorescent lighting where the amps waveform "lags" behind the voltage waveform. A typical site load would be in the region of 0.86 to 1PF
A PF greater than 1PF or also known as leading power factor is typically caused by a capacitive load. Power factor correction units use capactive leading power factor to balance other load lagging power factor
So for generators, the industry standard is to quote at a rating of 0.8 Power factor. This means the generator will operate at its Kilowatt output between 0.8 and 1.0pf without derating. Below 0.8PF would require derating of the equipment.