Paper Bowser is the incarnation of Bowser that appears alongside Bowser himself in Mario & Luigi: Paper Jam and it is the main antagonist of the game. Alongside various other paper characters, Paper Bowser came out of a book containing a whole paper world. Bowser was previously depicted as Paper Bowser in the Paper Mario series. He is the final boss with Bowser.
Not much else is known about Paper Bowser as of now but, from a trailer video, is shown not to get along with the main universe Bowser as they fought each other. In Paper Jam, Paper Bowser and Bowser lead many attacks against Mario and Luigi.