Good job :) I love the portal gun mechanic. Though it will be rather hard to implement.
For the design document itself - there is a lot of text and a lot of white space. Try transforming the wall of text into a combination of pictures and short descriptions or words. For example the puzzle could be better described with a picture of the possible solution.
As I said during the lab - try to think of the optimal path and even mark it in the design document. This should help with considering the level design itself as well. As it is, it's also rather hard to read which way the player should go on the map.
Use more colors in general! If there is a pressure pad and a box that should end up sitting on it, make note of it in the document! The pressure pad opens a door, color code it! I'm guessing you did the design in unity and took a screenshot, are the gizmos leftover telling me, the reader any information? If not, they should not be there.
In general, try to look at your design document after a day or two and see if there is any information that isn't clear from the document itself. Or even better - show it to a friend or family member, without explaining and let them ask questions. If those questions are along the lines of "What does this pressure pad do?" make note of it and add that information.