/var/cache/php-dompdf/fonts/Helvetica-BoldOblique.afm is in php-dompdf 0.6.0~beta3+dfsg0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/sdop/fontmetrics/Helvetica-BoldOblique.afm
/usr/share/sdop/fontmetrics/Helvetica-BoldOblique.afm