What is meant by pneumatic in reference to girls?

The word "pneumatic" is used by Huxley quite a lot in Brave New World. In its normal usage, it refers to something full of air, such as tires, for example. In the context of the story, however, it refers primarily to Lenina Crowe's body, with its shapely, voluptuous curves. It also applies to the gaping void between her ears. Most people are familiar with the term "airhead" to describe a stupid person, someone with a large, empty space where their brain should be, and that's what Huxley means in relation to Lenina.
But the meaning of the word is extended to this dystopian society as a whole. This is a place where vacuity is the order of the day, where people are not encouraged to think but to surrender themselves to the instant gratification of their senses. Everyone and everything in this brave new world is pneumatic: empty, full of air. In that sense, the comely figure of Lenina Crowe epitomizes precisely what citizens of the World State are expected to be.