A Physics problem...

We tend to think that, because we exert some effort to blow the air, that we are increasing the air pressure on the paper.  That is not the case.

Air pressure is the perpendicular force of air.  When we blow across the paper, we are creating horizontal motion in the air.  This horizontal motion actually reduces the pressure of the static air above the paper.  The horizontal motion interrupts the downward pressure of the static air.

The static air below the paper remains the same (the paper prevents the blowing air from affecting it).  Remember that force has both direction and magnitude (strength) and that the paper is subject to two forces - the air pressure from above and the air pressure from below.

When you blow across the paper, the air pressure below the paper is greater than the air pressure above the paper.  The cumulative value of these pressures is negative and the paper move upward.

Thank you so much!

You are very welcome!