You can use collision overlap to find exactly who is “touching” the item. When that happens, OnComponentBeginOverlap is triggered, and it gets information about the other object, in this case the player.
1: Start by adding a sphere collision to your pickupable item.
2: Make sure to enable OnBeginOverlap Events (scroll down in the details panel on the sphere collision.
3: OtherActor → Cast to player pawn to make sure it actually is a player pawn (now you have a reference to a specific player pawn), then pass the information on to whatever code that deals with removing the item from the world and applying the powerup effect on the player.
To get the player controller, simply get which controller is possessing this pawn.