When printing to PDF, the server will invoke the engine on the server to do that processing and generate the PDF. This involves having a separate process on the server access the web site. This will use the web server that's defined in the Configuration tab / Main Settings / Web Server Name. Because of network configuration or security issues (such as being in a DMZ), you may wish the server itself to refer to its own website using a different URL.

If the server cannot use the same URL as what's supplied in the Web Server Name entry of the Configuration tab (eg. DMZ), you can now supply a configuration file that will override the server name for use in, for example, creating PDF files and other server-server operations. 

Put a file called wkhtmltopdf.cfg into the Files folder of your installation. In that file, just put the text:

HostOverride=http://localhost

Replace the localhost part with the way you wish to reference the local server.