Paul, your code works fine, many thanks! But... It works only with small text samples. With entire book, your code is very very slow... and it doesn't works: there are false positive, saying that the document contains Unicode characters whereas it is false. In order to check that, we can compare using WinMerge the text with and without Unicode encoding. Then... I found the solution!!! I just have to save as text with and without Unicode and reload it and compare the results: it is very fast. Thanks anyway for helping.

P.S.: Your code says that for example character É is a Unicode character, this is ambiguous, because it can finally be saved using ANSI encoding. Unicode is really required for example for exotic characters like narrow no-break space (NNBSP : Alt+8239 : U+202F).

Exactly what do you expect with a large document? To complain about the speed betrays a totally unrealistic expectation. As even VBorNotVB said, the code:
Your fault-finding is rude! You can't reasonably expect a test for Unicode characters to say a given Unicode character isn't Unicode just because there's an ASCII equivalent to represent the same character (e.g. É). If you don't want to use a Unicode character when there's an ASCII equivalent, that's your responsibility, but don't go criticizing others when the solution is correct for the problem as described and any problems are due to your poor description of what you want to achieve.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]