In this article, I am going to explain you below terms
A Proxy Server is an intermediate server between a client application such as browser and the real server. Any request you make through your browser first go to proxy server then it forward it to real server. The same way, response given by real server is first captured by proxy server and then given to your browser.
Yesterday, I uploaded one of my ASP.NET MVC application on local IIS. I got above error message when I tried to access a web page that returns pdf file.