Just wanted to pitch in here as the person that keeps all the documentation on our website up to date. We only keep our own site up to date - we do not have agreements with any of these other sites mentioned to feed them with the latest version of any drawing. As for distributors, we feed our authorised distributors a list of links so they can link direct - if they are taking their own copy we can't guarantee it's up-to-date I'm afraid.
As an engineer I also would go straight to the manufacturer first whenever possible, for the same reasons. However, I do realise that not as many older or obsolete drawings are kept on manufacturer websites, and these sites may be useful for these documents. Other manufacturers may also have larger teams that can ensure documentation is kept updated at other sources.