[XeTeX] searcheable pdf with xetex

[jadolov k]

I am writing a paper with XeLaTeX, and I am using extensively small
caps letters and old styled numbers. But when I compile the PDF
output, all the strings shaped with this font features become non
searcheable, probably because they are not made of true glyphs but
only of glyph variants which do not correpond to any unicode
character.
So, how could I make them searcheable? Or, is it a price to pay for
using the glyph substitution via OpenType?

Best wishes,
Artemio Keidan