in PDF letters are placed by absolute positions. So replacing glyphs with different metrics will most probably result in broken documents rendering.

You should consider exporting to a textfile and doing the replacement there. (see Parsing PDFs by text position?)

Or to run a little script on the pasted text.

E.g. with emacs you could run a hook on every insert, which could do the transformation.

Or maybe ... did you check if your pdf reader allows to run hooks on copies... ?

> and impossible to know what letters it should be.

I doubt this could you give us some examples?