There is nothing special about PDFs. You just need the host name or IP address and the port number.
If the port number is not in the URL, the default is 80 for http and 443 for https.
High volume servers like Amazon's may have many IPs or floating IPs and you may struggle to add them all, but it depends how the firewall is set up.
Now, if it doesn't seem to work for Acrobat, and Acrobat is otherwise working, check the firewall logs to see what else is going on. It should not need any special access; in fact there cannot be any secret or special access to a file, it's all in the URL.