Is the pdf located in your Joomla install somewhere? Like images/ for example? Place your PDF in a folder inside your Joomla install then place a .htaccess file with the below contents in the same folder to htaccess protect it.
If you want to protect it using CBSubs then you need to install the CBSubs folderaccess plugin and specify the absolute path to the pdf folder. You then need to trigger the subscription at least once for it to create the files (subscribe to the plan, then login via frontend, it should protect it then).