I have for a while been researching nutrition and food, my new pup is being fed on pedigree puppy food she is a giant breed pup, should i trust the breeder, or change to something else??

Many breeders even if not using a particular brand all the time for all their dogs will rear pups onto a brand of food that is easily available to the puppy buyer, and is still  a decent food, usually one in the mid/upper price/quality bracket.

If you really want to change ask the breeder what else they would recomend.  I would always prefer to feed what the breeder has found works for their dogs, as it is tried and tested.

Pups grow and change so much at this stage that it is not the time to experiment.