Is PFC compulsory for AC/DC converters 75W and above?
EN61000-3-2 is a European standard which dictates PFC requirements. Most power supply manufacturers design in PFC so that there aren't any problems marketing the product worldwide.

PFC is also helpful if you want to operate with "universal" AC input (85-264VAC) as the down converter will see a constant input voltage (usually 400VDC) regardless of the input line voltage.

EN61000-3-2 also cites harmonic content limits in four broad categories of power supplies with explicit limits and test criteria. The 5% figure cited by Microchip is a rule of thumb that "should" allow you to pass harmonics testing, but doesn't replace a proper test with an accurate power meter and properly-controlled input AC.

Properly-working PFC makes the power supply look resistive to the mains (power factor as close to 1 as possible) which is why PFC requirements and line harmonic requirements go hand-in-hand.

In a nutshell, if the supply is greater than 75W, and has universal input, PFC is for all intents and purposes "required" and is a good feature to have even if you don't intend to sell in Europe.