/usr/share/sgml/dtd/linuxdoctr.dtd is in linuxdoc-tools 0.9.69-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/sgml/dtd/linuxdoctr96.dtd
/usr/share/sgml/dtd/linuxdoctr96.dtd