To use pressure-sensitive input with PhantomPDF, users must install Wacom drivers from here if they have not already.
Pressure-sensitive input is available only with the Comment > Pencil tool. And it will only work with a device that supports it, using its included touch pen. Some examples are the Microsoft Surface Pro, Wacom Cintiq, Dell Venue Pro, some Sony VAIOs, and some HP Touchsmarts. Generally these will be tablet or tablet-like devices with included styluses. If users have installed the above driver and pressure-sensitive input is not working, it is likely their devices do not support it.