Set the rectangular window of the page to be printed.
void pdfPrintSetRectangle(PDFHandle pdf, double llx, double lly, double urx, double ury)
This function sets the coordinates of the rectangular region (lower-left corner = llx,lly; upper-right corner = urx,ury) of each page to be printed. The coordinates are in PDF user space.

pdfPrintResetParams resets this to its default value, which is to print the whole page.

pdfPrintSetRectangle(pdf, 36.0, 36.0, 576.0, 756.0);