to get pure white backdrop for a person, you just need anything is white as background, also need space, lot of space, you need to blast the backdrop two stops over than your keylight.
For an example, your keylight is f8, if the exposure on white paper backdrop is also f8, it will turn out to be light grey. so you want pure white, your backround lights has to deliver at least f11 1/2.
or else you can always photoshop it