You don’t need Ajax if using iframes. The iframe makes a request to the src attribute for it’s content. Iframes are sort of problematic because requesting a post by its permalink will include all the theme chrome with the content. You’d need some sort of post template which only includes post content without theme chrome. Even then, these days iframes can run into CSP issues. As long as you control the source’s headers, CSP can be dealt with.
Alternately, use Ajax to fetch post content and insert it into a div container for the purpose, not an iframe. You could develop a custom Ajax handler to deliver post content, or use the REST API to get post data in JSON format. Your JavaScript would then extract the post content from all the post data, then set it as the div container’s inner HTML.
