Why does paper lift up when you blow across it?

If you blow air over a curved surface, it tends to stick to that surface; it's called the Coanda Effect. If the surface, in this case the paper, is bending downwards, it will pull that air downwards with it.

If air (black) is blown over a curved piece of paper it will tend to stick to the paper, to do this the paper must be pulling it down (blue) so the air must push the paper up (red)

If you push something, it pushes you so if the paper is pushing the air down, the air will push the paper up.

This is exactly the same principle as is used in aeroplane wings; they push air downwards, so the air pushes them up.

The Coanda Effect can also be seen in a 'Bernoulli blower', or by putting a table tennis ball in the air flow from a hairdryer. The air going past each side pushes on the ball, and these forces keep the ball in the stream of air.