[yihui]

Because PDF is for printing purposes. There could be many many problem if you copy and paste from PDF (white spaces being eaten, ligatures, curly quotes, en/em-dashes, ...; almost as bad as Word, except that PDF is beautiful). So don't copy from PDF, but from HTML instead. HTML is much more faithful in terms of preserving characters.

[nonbel]

So it is just an inherent limitation of pdfs? That's too bad, for some reason I find the html format less intuitive.