Since this article was published in 2011, nearly all the manufacture websites listed have had updates and the links provided no longer work.
Fortunately, we’ve found a brilliant, free online resource which has thousands of manuals for synths, digital pianos, modules, MIDI devices and other gear.
All the prominent manufacturers seem to be present. The site is fairly basic and there isn’t a search facility. You simply click on the nanufacturer’s logo from the front page, and then find the model number of the instrument you want the manual for.
Having used the site it seems pretty reliable. All the manuals are stored locally so unless the site itself goes down, there’s no worry about broken links.
