Question

why does a strip of paper rise after blowing over its surface

Answer 1

First the air blows by the paper causing some air in the triangle region to blow with this air (this is called entrainment). The air stream moves downwards towards the low pressure zone and the paper moves upwards towards this low pressure zone. The force goes from high pressure to low pressure.

Answer 2

When we blow air above the paper strip, low pressure is created above the strip, because high speed winds are always accompanied by reduced air pressure. Therefore, because of the higher air pressure below the strip, it is pushed upwards.