May I guess what happened?

My guess could be totally off. But is it possible that your manager doesn't know what you have been doing with the assignment? While you were doing the design, did you update him of your ideas, your approach and potential issues? Sometimes, although not ideal, a manager is ok with the situation that he is not clear about what a team memeber is doing, as long as that team memeber can handle all the issues and deliver as scheduled. However if anything goes wrong, the manager will be upset because it wasn't in his control. The manager might think that he could prevent this from happening if he was well informed.
In your case, is it possible that your manager was simply showing his frustation of not knowing what you have been doing, and he was taken by the surprise of the delay of the task? The whole thing wasn't really about you reading the document or not. I am sorry if my speculation is totally wrong, but it might help you see from a different angle.