Hi Paul, because our Pastry Flour has a much lower protein percentage (8%) as compared to our Unbleached All-Purpose Flour (11.7%), this means that the dough won't be as strong and will also be wetter (higher protein flours absorb more liquid than lower protein flours). Croissant dough needs a certain amount of strength to hold the butter and allow for proper rise and airiness, so I think you're better off sticking to the AP flour called for in this recipe. 

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