Printing to PDF using built-in Windows PDF driver without prompting for a filename requires a simple additional step:
 

C# / .NET

TGIS_PrintManager printman = new TGIS_PrintManager();
PrintDocument printdoc = new PrintDocument()
{
  PrinterSettings = new PrinterSettings()
  {
    PrinterName = "Microsoft Print to PDF",
    PrintToFile = true,
    PrintFileName = @"c:\Temp\myfile.pdf"
  )
};
TGIS_Printer printer = new TGIS_Printer( printdoc );
printman.Print( GIS, printer );

Delphi /VCL 

var
  hprinter : THandle;
  sdevice,
  sdriver,
  sport    : array[0..80] of Char;
  oprinter : TGIS_Printer ;
  oprintman: TGIS_PrintManager ;
begin
  Printer.PrinterIndex := Printer.Printers.IndexOf('Microsoft Print to PDF');
  Printer.GetPrinter( sdevice, sdriver, sport, hprinter ) ;
  Printer.SetPrinter( sdevice, sdriver, 'C:\Temp\myfile.pdf', 0);
  
  oprintman := TGIS_PrintManager.Create ;
  try
    oprinter := TGIS_Printer.Create( Printer ) ;
    try
      oprintman.Print( GIS, oprinter ) ;
    finally
      oprinter .Free ;
    end;
  finally
    oprintman.Free ;
  end ;
end;