Is it possible to download a PDF/HTML snapshot of the Blender Manual from the Wiki and integrate into Blender's help menu?
You cannot directly download the blender wiki… well not as normally as you are asking, there are ways (and even if you did get it, as far as implementing or getting its content into Blender, that's another story).
You can reach the manual from inside Blender however and some documentation functionality is already implemented somewhat..
Hover over a button and you should see a short description etc.. This can be useful for getting operator names.
You can also just grab a zip of the python reference for offline viewing directly from the front page.
There are many download managers (such as wget) that can crawl the wiki and make a local copy (could also be scripted via python). But make sure your use case is covered by fair use.
partial answer (pdf snapshot):
I have done wiki manual > pdf several times in the past.
last update is form july 1st 2013, so quite up to date I hope...
I dont'know how to link this pdf from blender itself but it would be great...
last conversion is here https://archive.org/details/BlenderWikiManualPdf20130701
if you care, details, updates and future versions (links) will be posted around here, atm: http://amrc.altervista.org/nuova-versione-del-blender-wiki-manual-pdf/
--dec 2015 edit--
I even forgot I wrote this answer in 2013... : latest version of that PDF was november 2014, https://archive.org/details/BlenderWikiPDFManual then the wiki freezed...
Now, a year later, there is an official downloadable html manual from the manual page at http://www.blender.org/manual, the direct link atm is http://www.blender.org/manual/blender_manual.zip. A PDF version is (weirdly) still to be generated from that manual system, but I'll try soon to again generate periodically a PDF from this official html for anyone to share, somewhere.