How do I export PDF to Black and White tif file?

How can I convert PDF page to 8 bpp grayscale or monochrome TIFF?

PDFNet built-in rasterizer renders images to 24-BPP RGB Color image or
8-BPC GrayScale images.
Rendering to monochrome (1 BPC) would involve significant loss of
image quality and is as a result not directly supported at the moment.

You can produce 8 BPC grayscale image you could specify 'ColorSpace'
parameter as a hint in the pdfdraw.Export() method. For example:

// In C++
Dict enc_param;
enc_param.Put("ColorSpace", new Name("Gray")); pdfdraw.Export(page,
filename, "TIFF", &enc_param);

// In C#
Dict enc_param = Obj.CreateDict();
enc_param.Put("ColorSpace", Obj.CreateName("Gray"));
pdfdraw.Export(page, filename, "TIFF", enc_param);

To export monochrome images, you could use pdfdraw.GetBitmap() and
then convert the System.Darwing.Bitmap to monochrome bitmap before
saving to TIFF (for an example of how this can be done, please see