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I've noticed that when using the MinionPro package, some glyphs like ligatures and oldstyle figures would be mapped to another character.

For example, compiling

\documentclass{article}
\usepackage[opticals]{MinionPro}
\begin{document}
123 Th fj fi ffj
\end{document}

using pdfLaTeX yields a PDF document that looks correct enter image description here

but if copied, it actually yields other glyphs like Ïth û ö.

I realise that this could be solved using glyphtounicode, but I'm thinking of how this property can be used to actually prevent plagiarism if I make all the glyphs map to wrong characters.

How can I achieve this using either pdfLateX or XeLaTeX? Is it as simple as creating a randomised custom glyphtounicode?