qt中把txt文件转换为pdf格式文件出错，高人帮忙,该如何处理

void TextEdit::on_action_Font_triggered()
{
    if(activeChildWindow()==NULL)return;
    bool ok;
    QFont font = QFontDialog::getFont(&ok,activeChildWindow()->font(),this);
    if(ok)
    {
        activeChildWindow()->setCurrentFont(font);//current可以undo
        //activeChildWindow()->setFont(font);
    }
}