/usr/lib/ruby/ is in ruby-pdf-core 0.6.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/pdf/
/usr/lib/ruby/vendor_ruby/pdf/core/
/usr/lib/ruby/vendor_ruby/pdf/core.rb
/usr/lib/ruby/vendor_ruby/pdf/core/annotations.rb
/usr/lib/ruby/vendor_ruby/pdf/core/byte_string.rb
/usr/lib/ruby/vendor_ruby/pdf/core/destinations.rb
/usr/lib/ruby/vendor_ruby/pdf/core/document_state.rb
/usr/lib/ruby/vendor_ruby/pdf/core/filter_list.rb
/usr/lib/ruby/vendor_ruby/pdf/core/filters.rb
/usr/lib/ruby/vendor_ruby/pdf/core/graphics_state.rb
/usr/lib/ruby/vendor_ruby/pdf/core/literal_string.rb
/usr/lib/ruby/vendor_ruby/pdf/core/name_tree.rb
/usr/lib/ruby/vendor_ruby/pdf/core/object_store.rb
/usr/lib/ruby/vendor_ruby/pdf/core/outline_item.rb
/usr/lib/ruby/vendor_ruby/pdf/core/outline_root.rb
/usr/lib/ruby/vendor_ruby/pdf/core/page.rb
/usr/lib/ruby/vendor_ruby/pdf/core/page_geometry.rb
/usr/lib/ruby/vendor_ruby/pdf/core/pdf_object.rb
/usr/lib/ruby/vendor_ruby/pdf/core/reference.rb
/usr/lib/ruby/vendor_ruby/pdf/core/renderer.rb
/usr/lib/ruby/vendor_ruby/pdf/core/stream.rb
/usr/lib/ruby/vendor_ruby/pdf/core/text.rb