Two options you might consider are Textile and Markdown.  Not sure
about support for rendering PDF.  Also, take a look at Instiki (wiki
clone) -- it supports both Textile and Markdown, and also can export
pages to PDF (though it may be using a separate library for PDF
rendering).

Textile:
http://www.textism.com/tools/textile/
Textile reference:
http://hobix.com/textile/
Ruby implementation (RedCloth):
http://www.whytheluckystiff.net/ruby/redcloth/

Markdown:
http://daringfireball.net/projects/markdown/
Ruby implementation (BlueCloth):
http://bluecloth.rubyforge.org/

Instiki:
http://instiki.org/show/HomePage

Cheers,
Ken