to bring out the sick into the streets, and to put them on cots and mats, that of Peter coming, at least the shadow might envelop some of them.
The shadow of itself is nothing. The idea is that by putting people on the street, as/if Peter walked by he would be in close proximity and that would be near enough for the holiness of God IN Peter to effect healing.
Now many signs and wonders were happening among the people by the hands of the apostles. And they were all with one accord in Solomon’s Colonnade. 13Now none of the rest dared to join them, but the people were magnifying them. v12-13
They thought that just being near to an Apostle was enough for a miracle to happen. The "shadow' is merely an evidence (in their minds) of Peter's physical closeness to them.
The whole narrative is similar to, "if I just touch his cloak" Matt 9:21
Did the cloak have special powers? No, but the wearer did, just as Peter did, but not his 'shadow'.