Thanks ChrisW67. It worked. Code below:
Qt Code: 
  1. QPrinter impressora(QPrinter::HighResolution);
  2.  
  3. QTextDocument document;
  4.             document.setHtml(html);
  5.             if(matriz == true)
  6.             {
  7.                 QTextCursor cursor(&document);
  8.  
  9.                 QPalette palette;
  10.  
  11.                 palette.setColor(backgroundRole(), QColor(255, 255, 255));
  12.                 this->ui->tab_3->setPalette(palette);
  13.                 this->ui->tab_3->setAutoFillBackground(true);
  14.  
  15.                 QPixmap pm = this->ui->tab_3->grab();
  16.                 pm = pm.transformed(QTransform().rotate(270));
  17.  
  18.                 cursor.movePosition(QTextCursor::End, QTextCursor::MoveAnchor);
  19.  
  20.                 QImage img = pm.toImage();
  21.                 img = img.scaledToHeight(img.height()-60);
  22.                 cursor.insertImage(img);
  23.  
  24.                 this->ui->tab_3->setAutoFillBackground(false);
  25.             }
  26.  
  27.             document.print(&impressora);
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