/usr/lib/petscdir/3.1/index.html is in petsc3.1-doc 3.1.dfsg-11ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/petscdir/3.1/../../../share/doc/petsc3.1-doc/docs/index.html
/usr/lib/petscdir/3.1/../../../share/doc/petsc3.1-doc/docs/index.html