One well known problem with using the font Linux Libertine (presumably other fonts too), is that its many ligatures are not recognized in the resulting pdf-file, so that it's not possible to search for or copy a ligature such as 'Th'.
One way of fixing this is addressed in the question Make ligatures in Linux Libertine copyable (and searchable)
But the fix addressed there doesn't work with XeTeX. Compiling Ulrike's MWE will only result in an error message
> Undefined control sequence. l.7 \pdfglyphtounicode
So what is the best fix for XeTeX?