Ducks can and will eat popcorn but popcorn is not the best food to feed ducks.
Ducks can potentially get choked on popcorn seeds so I would not recommend feeding your own ducks or any other ducks popcorn.
Bread and other foods that do not pose a choking hazard for the ducks would be much better.
I've heard that you should not feed ducks bread but we have a park here that allows people to feed the ducks and it's okay to feed those ducks bread and it has never harmed them.
Duck feed is best for ducks but feeding them a bit of popcorn or bread is okay.
Crackers are also good to feed ducks and they provide the duck a bit of nutrition as well.