Science

這是因為書幫紙張抵擋了空氣阻力，

若紙張比書大..那麼2者就會在墜落途中分開。

The answer given by the above post is not correct.
The reason is that once the book falls faster than the paper, there will be a gap between the paper and the book. If this happens, the separation between the two (book and paper) creates a partial vacuum between them. Hence, air pressure above the paper forces the paper to fall faster until it sticks to the book again.