//Add to functions.php
function show_post($path) {
  $post = get_page_by_path($path);
  $content = apply_filters('the_content', $post->post_content);
  echo $content;
}

//add to your page/post/template file
<?php show_post('about');  // Shows the content of the "About" page. ?>
<?php show_post('category/postname');  // Shows content of a Post page. ?>

By adding this function to ones functions.php and then a call in ones page.php, single.php, home.php, index.php or custom template file. You can include on page within another.

WordPress Include Page or Post within another Page or Post via Templates

PHP