Pet ducks cannot bite you because ducks do not have teeth so pet ducks do not bite.
However pet ducks or any other ducks can peck you with their beaks which may break the skin and cause bleeding.
But most ducks including pet ducks will not bite you unless you do harm to them first or annoy the duck first.
Ducks are pretty friendly and it's rare that a duck tries to attack someone.
Ducks cannot bite your finger off or bite any part of your body off.
The reason ducks cannot bite your finger off is the ducks do not have teeth and without teeth they cannot bite something off.
A duck can still attack a human though and may cause some bleeding if they pierce your skin with their beak which is possible.
However most ducks will not attack you or try to bite you as long as you're friendly to the duck.
I've been around ducks and most of the ducks are pretty friendly.