Thanks for your answers; in the end it works with:

string fontPath = System.IO.Path.Combine(cale, "segoeui.ttf");
BaseFont unicodeFont = BaseFont.CreateFont(fontPath, BaseFont.IDENTITY_H, BaseFont.EMBEDDED); .... pdfStamper.AcroFields.SetFieldProperty(camp, "textfont", unicodeFont, null);

In PDF, the font was already Segoe UI. And for previous results, I was using Adobe Reader, Foxit Pdf Reader and Firefox browser.