Origin of the false document technique
The technique is chiefly associated with postmodernism, but is both older than that movement, and also encompasses art pieces and activities outside of the scope of art usually considered part of any "artistic movement." One of the earliest examples of the technique is the 18th century French novel The Nun, by Denis Diderot. It was begun originally not as a work for literary consumption, but as an elaborate practical joke aimed at making a wealthy philanthropist give support to a spurious cause.
It seems to grow out of the epistolary novel but has more in common with the newspaper serial from which it draws most of its technique. The conceit is most commonly used where a heightened sense of authenticity is required for the desired effect of the story to be maintained. Blurring the line of reality and fiction is an important component of horror, mystery, detective and fantasy narratives because they wish to engender in a reader a sense of wonder, and of danger, both of which need to feel more present then a typical narrative form would allow. For this reason, false documentary techniques have been in use for at least as long as these literary genres have been around. Frankenstein draws heavily on a forged document feel, as does Dracula, Dr. Jekyll and Mr. Hyde and many of the works of Jules Verne and H.G. Wells. Vladimir Nabokov's Pale Fire is a particularly elaborate variation.