Reference manual is not available in the help menu of the rgui

Re-install, and this time check the box to install
that manual. But as 

Dear Mr. Murdoch, thank you for the answer, apparently
there is no other solution than a re-installation.

Peter says, it's not really very useful, it's just a
collection of man pages from the base packages.

... I want to have the reference manual only because
my eyes like it more to read pdfs then text in the
console (and with the search function I am as quickly
as in the other case).