Force printing in monochrome.
void pdfSetPrintForceMono(PDFViewerHandle viewer, BOOL mono)
If this function is called with mono = true, XpdfViewer will generate monochrome output, even if the printer appears to be color.

This setting is only relevant for GDI printing, i.e., for non-PostScript printers, or if pdfSetPrintForceGDI is in effect. It is ignored when XpdfViewer generates PostScript code.

pdfSetPrintForceMono(pdf, TRUE);