pdfEnableDeviceNOverprintPreview
Enable or disable DeviceN+overprint preview mode.
void pdfEnableDeviceNOverprintPreview(PDFViewerHandle viewer, BOOL on)
If this is enabled, XpdfViewer renders the PDF file in DeviceN (CMYK +
spot colors) mode, with overprint preview. This is useful for
prepress previews. The default value is disabled.
C:
pdfEnableDeviceNOverprintPreview(viewer, TRUE);