How to save pdf formet in SaveFileDialog?

public static bool SaveDIBAs( string picname, IntPtr bminfo, IntPtr pixdat )
		{
		SaveFileDialog sd = new SaveFileDialog();

		sd.FileName = picname;
		sd.Title = "Save bitmap as...";
		sd.Filter = "Bitmap file (*.bmp)|*.bmp|TIFF file (*.tif)|*.tif|JPEG file (*.jpg)|*.jpg|PNG file (*.png)|*.png|GIF file (*.gif)|*.gif|All files (*.*)|*.*";
		sd.FilterIndex = 1;
		if( sd.ShowDialog() != DialogResult.OK )
			return false;

		Guid clsid;
		if( ! GetCodecClsid( sd.FileName, out clsid ) )
			{
			MessageBox.Show( "Unknown picture format for extension " + Path.GetExtension( sd.FileName ),
							"Image Codec", MessageBoxButtons.OK, MessageBoxIcon.Information );
			return false;
			}
		
		IntPtr img = IntPtr.Zero;
		int st = GdipCreateBitmapFromGdiDib( bminfo, pixdat, ref img );
		if( (st != 0) || (img == IntPtr.Zero) )
			return false;

		st = GdipSaveImageToFile( img, sd.FileName, ref clsid, IntPtr.Zero );
		GdipDisposeImage( img );
		return st == 0;
		}