Do you use the shellPrint command from the pdf24-doctool? This shellprint command simply uses ShellExecute from windows with the printo or print operation. If the printto operation is not registered in windows, then we use therint operation as fall back and there is can be that the print out fails. In this case we do not exactly know whether the the file is printed correctly on the pdf printer or not and therefore we show this little message box. I can have a look at the code to search for options to prevent this message if this would help you.