"QPdfWriter" created by "QIODevice" can't change page size

QPdfWriter::QPdfWriter(const QString &filename)
    : QObject(*new QPdfWriterPrivate),
      QPagedPaintDevice(new QPdfPagedPaintDevicePrivate(d_func()))
{
    Q_D(QPdfWriter);

    d->engine->setOutputFilename(filename);

    // Set QPagedPaintDevice layout to match the current paint engine layout
    devicePageLayout() = d->engine->pageLayout();
}

QPdfWriter::QPdfWriter(QIODevice *device)
    : QObject(*new QPdfWriterPrivate)
{
    Q_D(QPdfWriter);

    d->engine->d_func()->outDevice = device;

    // Set QPagedPaintDevice layout to match the current paint engine layout
    devicePageLayout() = d->engine->pageLayout();
}