If the manual is designed to be used online (i.e. is effectively a series of web pages), you could try something like QuadSucker ( see here).
It can be a bit fiddly to use and you would need to set it up to use relative links so that when it has downloaded the website you can run it (the website) from your local PC.
Quadsucker has limitations (e.g. won't download from https site). And I would set it up to not download anything from external links (i.e external to the website you are downloading).
Lastly, in using something like Quadsucker you should be aware of potential copyright issues.