Setting page size QPdfWriter does not have any effect

#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QFile file("C:/test2.pdf");
    file.open(QIODevice::ReadWrite);
    QPdfWriter pdf(&file);
    pdf.setPageSizeMM(QSize(100,50));

    QPainter painter(&pdf);
    painter.fillRect(100,100,200,200,Qt::red);
    painter.end();

    file.close();

    return 0;
}