Process p = new Process( );
p.StartInfo = new ProcessStartInfo( )
{
    CreateNoWindow = true,
    Verb = "print",
    FileName = path //put the correct path here
};
p.Start( );

public static bool Print(string file, string printer) {
     try {
        Process.Start(
           Registry.LocalMachine.OpenSubKey(
                @"SOFTWAREMicrosoftWindowsCurrentVersion" +
                @"App PathsAcroRd32.exe").GetValue("").ToString(),
           string.Format("/h /t "{0}" "{1}"", file, printer));
        return true;
     } catch { }
     return false;
  }

Process.Start("LPR -S printerdnsalias -P raw C:filesfile.pdf");