Ok, so most followers in this game are able to hold a limited amount of items when you trade with them, but can hold an infinite number of items with a glitch (I'm not really sure if it is a glitch or if it was intentional). You have to ask them to do something and then tell them to pick up an item or take all from a chest/container. However, I can't get Serana to do it because she doesn't have the "I need you to do something." dialogue option. Is there a way to get her to pick up items?
If you look at her feet and hold down the select button for a few seconds, then let go, it will give you the odd, x-shaped cursor that you get when you ask followers to do something. You can then move to the item/container and press the select button again. You can then exit the action the same way you would with the infinite inv. glitch. This works with all followers that I have encountered. I have only done this with the PS3 version (although I didn't have Dawngaurd on there, so I couldn't actually do it with Serana... but it worked with every other follower I actually tried) and PS4 Skyrim Special Edition.