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void MainWindow::on_btnPrint_clicked()
{
    QPrinter printer(QPrinter::HighResolution);
    printer.setOutputFormat(QPrinter::NativeFormat);
    printer.setOrientation(QPrinter::Portrait);
    printer.setPageMargins(20, 10, 20, 10, QPrinter::Millimeter);

    QPrintPreviewDialog preview(&printer);
    connect(&preview, SIGNAL(paintRequested(QPrinter *)), SLOT(preview(QPrinter *)));

    preview.exec();
}
//**********************************************************
void MainWindow::preview(QPrinter *printer)
{
    QPainter painter(printer);
    painter.setRenderHints(QPainter::TextAntialiasing);

    QString html = "<u>Подчеркнутая строка</u>";
    QFont font;
    font.setFamily("Times New Roman");
    font.setPointSize(20);
    painter.setFont(font);

    QTextDocument doc;
    doc.setDefaultFont(font);
    doc.documentLayout()->setPaintDevice(printer);
    doc.setPageSize(QSizeF(printer->pageRect().size()));
    doc.setHtml(html);
    doc.drawContents(&painter);
}