To output a PDF pages to the printer it is almost the same as displaying the pages on the screen. Windows system treats both screen and printer as display devices. So you can get a DC (Device Context) for a printer, and then you can use the same FPDF_RenderPage function to output to the printer.
The only difference would be for printing you need to deal with documents and pages. We don’t discuss how to get a printer DC here, please refer to WIN32 documents or documents about your development tool.
Here is an example on how you can print a loaded page:
// Get a printer DC
hDC = CreateDC("WINSPOOL", "Printer Name", NULL, NULL);
// Start a printer job
// Start a new printing page
… // we need to calculate the page size here
// Now we can render the page to the printer
FPDF_RenderPage(hDC, pdf_page, 0, 0, size_x, size_y, 0, 0);