[NTG-context] Unreadable characters in embedded PDF file

Hello all, Often I include external PDF files into PDF document generated by ConTeXt. When I create a PDF document containing text in Scribus (or other application) and then I put it to my PDF file (generating by ConTeXt), then in final PDF document are unreadable characters. Is there a problem in Scribus character encoding or the fact that the fonts contained in the included PDF file ConTeXt unknown? Thanks for the advice. Jaroslav