Most powdered peanut butters contain the ingredient xylitol which is a sugar substitute used in candy, gum some peanut butters.
All the powdered peanut butters I've seen in the stores have the ingredient xylitol although there might be some made without xylitol but I've never seen any.
The ingredient xylitol is highly toxic to dogs when ingested and can kill the dog after eating it so if in doubt don't feed them the powdered peanut butter.
But if you can find some powdered peanut butter made without that ingredient xylitol then you can safely feed it to your dog.