As user ckan:
. default/bin/activate
pip install ckanext-pdfview
Edit the file /etc/ckan/default/production.ini and add the plugin:
ckan.plugins = [...] pdf_view
Also make sure the resource_proxy plugin is enabled in the plugins line if you want to render PDF files which are not located in the same server as CKAN.
Refer to the official extension page for further details at https://github.com/ckan/ckanext-pdfview