If you encounter empty text lines in PDFs exported from DokuWikis PDF Plugin dw2pdf, here is something that helped me workaround it.
With only Google Chrome showing the PDFs content correct, I tried Evince and Acrobat under Windows. Under Windows I got the feedback, that the font dejavusanscondensed was not found. So as a workaround, I can currently tell you, that the following worked:
$this->sans_fonts = array('dejavusanscondensed','dejavusans','freesans',...);
There you have to put helvetica in front of this list, so it is used before dejavusanscondensed.
$this->sans_fonts = array('helvetica','dejavusanscondensed','dejavusans',...);
After this little change, Evince and Acrobat are now able to show the invisible text.