Qt generated pdf shows extra line in dotted line in Adobe Reader

The attached html file shows a dotted line in webkit. When export it into a pdf file using the following code, the file shows an extra line in Adobe reader.
(It doesn't show this line in Evince or Foxit reader.)

QPrinter printer;
printer.setOutputFormat(QPrinter::PdfFormat);
printer.setOutputFileName("./output.pdf");
mainFrame()->print(&printer);