RDoc2PDF v0.1 has been released on Rubyforge:
http://rubyforge.org/projects/rdoc2pdf/
RDoc2PDF installs a very basic PDF generator for RDoc. The output is
generated by PDF-Writer, so a recent install of that module is
required. Because of the way RDoc searches for generator plugins I
don’t think a gem version of RDoc2PDF can be made so you will need to
install using the setup.rb script included with the release:
ruby setup.rb all #See setup.rb docs for further options
Once installed you can generate PDFs by calling rdoc with the ‘-f’
option set to ‘pdf’. Without the ‘–one-file’ option each class is
placed in a separate pdf, with it all classes are included in a
single pdf called ‘doc.pdf’ which is written to the current working
dir):
rdoc -f pdf --one-file foo.rb
Some RDoc formatting commands such as typewriter font and includes
are not supported, but it seems to run reasonably on most of the
stdlib docs I have tested it on. Hyperlinks to method names and other
fancy features you have in the HTML generator are also not supported.
The current code is based on the Ri generator and is pretty hacky to
be honest. I won’t have time to really work on this further, but
would be happy to accept bug reports/patches - I’d prefer patches 
Dr Alex G.
Post-Doctoral Researcher
Bioinformatics Center
Institute for Chemical Research
Kyoto University
Gokasho, Uji, Kyoto 611-0011
Japan