[XeTeX] Ligatures in copy and paste

[John Haiducek]

I find that when I generate a pdf with ligatures, the Unicode characters
for the ligatures get copied in copy and paste operations, rather than
the original characters which the ligatures represent. 

For instance, if I have 'find' in my LaTeX file, then the output appears
as 'nd'. That's desirable for the display in the pdf, but when I copy
and paste from the pdf into some other program I'd like to get the
original characters back. 

I've seen this done in other XeTeX-generated pdf's, but I can't figure
out what's different in the TeX sources between those files and mine.
Can anyone explain this to me?

John Haiducek