I just spent quite some time at ↑Replacementdocs, clicking through 66 pages containing 1352 game-manuals for PC alone (all in .pdf format—many more for other platforms), and in the end downloaded 52. “replacementdocs.com pledges to bring you only high quality scanned images of instruction manuals in their full, original format with all original artwork and other graphical elements intact. There will be no text-only documentation unless that is how it was originally released. There will also be no conversion to monochrome images or any other severe quality degradation.” Another worthwhile resource for computergame historians is ↑The Doom Bible, being the original design document.
replacementdocs via entry at gamersgame