/usr/share/doc/pd-pdp/introduction is in pd-pdp 1:0.14.1-3build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/pd-pdp/../../../lib/pd/extra/pdp/introduction
/usr/share/doc/pd-pdp/../../../lib/pd/extra/pdp/introduction