Why does the voltage across the DC-link capacitor in a boost PFC circuit usually equals 400 Volt?
A PFC boost controller attempts to make the current taken from the AC mains "look" like it's being drawn by a resistive load. So, at low parts of the input supply voltage waveform, the PFC controller transfers low amounts of energy to the output capacitor and this means low current taken from the incoming supply. 

At higher parts of the incoming voltage waveform, more energy is transferred with the highest amount of energy being transferred (not surprisingly) at the peaks of the waveform.

This makes it look like a resistive load and the power companies are happy.



So, PFC turns a rather peaky current waveform into a much more sinusoidal waveform with smaller peaks: -



You should be able to see that energy is taken throughout most of the waveform cycle as opposed to just the peaks (as in a conventional rectifier and smoothing capacitor).

Given that the peak of the incoming voltage waveform (under maximum supply conditions) might be 265 V x \$\sqrt2\$ (375 volts), the PFC boost controller has to output a voltage higher than this. If it didn't then it would shut-down at the high points of the voltage cycle and this would wreck its ability to control power factor.

So, 400 volts seems a good number to aim for and you'll find a lot of capacitors rated at 450 V for that reason.