Ray Bradbury's classic science fiction book, Fahrenheit 451, is about burning books, so its title gives the temperature at which plain paper spontaneously combusts. According to Gerry Ring, Professor of Paper Science at University of Wisconsin-Stevens Point, "Paper is essentially cellulose. Coated or saturated papers may have combustion temperatures dramatically different."
Another paper scientist, Dick Hoyland, at University of Manchester for Industrial Science and Technology, says "I do not know the answer but would guess that it will vary considerably depending on the type of paper. I checked a few books but nothing on combustion of paper. The only thing I found was that the temperature required for spontaneous combustion of wood varies from 273 to 500 degC (this was in a book published in 1954!).