/usr/share/doc/puredata-doc/6.externs is in puredata-doc 0.43.0-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/puredata-doc/../../../lib/puredata/doc/6.externs
/usr/share/doc/puredata-doc/../../../lib/puredata/doc/6.externs