[XeTeX] how to do (better) searchable PDFs in xelatex?

[Joe Corneli]

Hello:

Having looked with Google and found this pointer -

http://tex.stackexchange.com/questions/67848/can-pdf-search-find-words-with-ligatures-in-xelatex-documents

However, if I extend the MWE there slightly, I can find "prefix", but
not "quantitative".  (My PDF reader is Evince on Ubuntu 12.04.)

\documentclass{book}

\usepackage{amsmath}
\usepackage[MnSymbol]{mathspec}
\setmathsfont(Digits,Latin,Greek)[Numbers={Lining,Proportional}]{Linux
Libertine O}
\setminwhitespace
\setmathrm{Linux Biolinum O}

\usepackage{xunicode}
\defaultfontfeatures{Mapping=tex-text}
\setmainfont{Linux Libertine O}

\begin{document}
Quantitative/Prefix.
\end{document}

Any help would be appreciated!

Joe