/usr/share/php/dompdf/lib/fonts/DejaVuSerif.ttf is in php-dompdf 0.6.1+dfsg-2ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/php/dompdf/lib/fonts/../../../../fonts/truetype/dejavu/DejaVuSerif.ttf