Zoom to a specific rectangle.
void pdfZoomToRect2(PDFViewerHandle viewer, int page, double xMin, double yMin, double xMax, double yMax)
This function changes the zoom factor and scroll position so that the specified rectangle just fits in the window.

The rectangle is given in PDF coordinates (not pixels). This is the same coordinate space used by pdfGetCurrentSelection2, for example.

int page; double xMin, yMin, xMax, yMax; if (pdfGetCurrentSelection2(viewer, &page, &xMin, &yMin, &xMax, &yMax)) { pdfZoomToRect2(viewer, page, xMin, yMin, xMax, yMax); }