this has already been asked and answered here:
How can I send a file document to the printer and have it print?
The code that was used:
private void SendToPrinter()
{
ProcessStartInfo info = new ProcessStartInfo();
info.Verb = "print";
info.FileName = @"c:\output.pdf";
info.CreateNoWindow = true;
info.WindowStyle = ProcessWindowStyle.Hidden;
Process p = new Process();
p.StartInfo = info;
p.Start();
p.WaitForInputIdle();
System.Threading.Thread.Sleep(3000);
if (false == p.CloseMainWindow())
p.Kill();
}
it basicly opens a "hidden" pdf-reader, tells it to print, waits for it to finish then close it down