Question

if all objects fall at the same velocity (9.8m/s) but different objects are effected by air resistance, why when a paper and a book are the same size, they have different air resistances?

Answer 1

Air resistance in case of a book is too low compared to its weight, so it falls down rapidly. But a paper's weight is very low and comparable to its air resistance, so the papers falls slowly. Air resistance, if the upthrust of air is considered, depends on the density of air, the volume of the object and acceleration due to gravity\[Buoyant force = V rhog\]
So different objects experience different air resistances, but since the difference is too small to be measured, we see all objects(unless too light) to be reaching the floor at the same time.
P.S. In your question, you must have said, all objects fall with the same 'acceleration' not velocity