Enable the plugin by editing the file in config/config.rb:
AppConfig[:plugins] >> 'yale_pdf'
Also add the following to config/config.rb:
AppConfig[:pui_page_actions_print] = false
This will turn off the default PDF page action that is set to appear in the PUI.
Yale plans to use something like this plugin so that the PDF links in the PUI will go to statically generated PDF/UA files, which will be available on another web server. These PDF/UA files will be generated and updated nightly by Hudson Molonglo's ArchivesSpace Export Service plugin: https://github.com/hudmol/archivesspace_export_service
For now, the expectation is that you would change the PDF root URI directly within the ERB file.
Restart the application.