Storing anything inside an electrical panel seems like a bad idea. Why would you want to do that? The only thing that should be stored in an electrical panel is its panel schedule and perhaps a partial single line. But these are normally affixed to the inside of the panelboard door. I doubt that UL included a roll of drawings inside the panel enclosure when they did their testing.
I have never seen prints stored in a breaker panel, but, while it might not be a good idea, it is common to find prints stored in control panels. There are often door pockets for paper work in these panels. A number of plants that I have worked in have stored a set of panel drawings in an 11x17 format in these pockets.
