Paper

Paper was a material made from wood or a similar materials. Paper was often used as something to write on. (TV: Human Nature, Silence in the Library / Forest of the Dead)

Psychic paper was a blank, white card that had special properties. When shown to a person, it could usually induce them to see whatever the user wished them to see printed on it. This was commonly used by the Doctor, particularly in his ninth, tenth and eleventh incarnations. (TV: The Empty Child, New Earth, Army of Ghosts, The Shakespeare Code, The Unicorn and the Wasp, The Lodger, A Christmas Carol, et al.) Suggestibility paper was a material similar to psychic paper. (COMIC: Fugitive)

The Tenth Doctor met a group of Cybermen while holding a sheet of A4 paper on a stick in the place of a plain white flag, which he jokingly apologised for, to surrender to them. (TV: Doomsday)

When the Tenth Doctor and Donna Noble arrived at the Library and were surprised to find that, despite no one being visible, 1,000,000,000,000 life forms were detected there, Donna suggested that the books might be alive. They weren't books, but in fact the Vashta Nerada, who were hatched in the Library's books, using them in place of a forest. (TV: Silence in the Library, Forest of the Dead)