Set the matte color.
void pdfSetMatteColor(PDFViewerHandle viewer, COLORREF color)
This function sets the matte color, i.e., the color used for background outside the actual page (surrounding pages that are smaller than the window, and between pages in continuous view mode).

The default matte color is a medium gray.

/* change the matte color to white */ pdfSetMatteColor(viewer, RGB(255, 255, 255));