Change the page rotation.
void pdfSetRotate(PDFViewerHandle viewer, int rotate)
This function changes the current rotation angle. The angle is in degrees and can be 0 (the default), 90, 180, or 270. Normally, a PDF file specifies the correct rotation for each page (e.g., 90 degrees for landscape pages) - XpdfViewer handles this automatically, and the rotate property isn't needed. The rotate property is primarily useful with misconfigured PDF files that don't have the correct page rotation set.
pdfSetRotate(viewer, 90);