I believe you can do Western European diacritics with plain ANSI (I haven't tried it).
HPDF_Doc pdf = HPDF_New();
HPDF_UseUTFEncodings(pdf);
HPDF_SetCurrentEncoder(pdf, "UTF-8");
const char *ret = HPDF_LoadTTFontFromFile (pdf, faceName, HPDF_TRUE); // embedding Need 'HPDF_TRUE' for UTF-8 to work
HPDF_GetFont(pdf, ret, "UTF-8");