the_posts

/**
 * Require more work but does the job.
*/
class Posts {
    const CATCH_POST_PRIORITY = -32767;
    const TEXT_TO_CHANGE = 'my-plugin-text-needs-to-be-manipulated';
    /*
     * Manipulate our plugin's text\data
     * @param array $posts
     * @return array
     */
    public function manipulate($posts){
        $newPosts = array();
        foreach ($posts AS $post) {
            if (false === strpos($post->post_content,self::TEXT_TO_CHANGE)
                 || !self::IsPreviewMode($post->ID)){
                $newPosts[] = $post
                continue ;
            }

            //remove new lines
            //TODO: extract your data to your HTML (real plugin uses a View object and render's the data)
            $html = str_replace("\n","",file_get_content("some.html"));
            //assign back to post content
            $post->post_content = str_replace(self::TEXT_TO_CHANGE,$html,$post->post_content);
            $newPosts[] = $post;
        }
        return $newPosts;
    }

    /**
     * Check if we are in preview mode of current post id
     * @param int $postID
     * @return bool
    */
    private static function IsPreviewMode($postID){
        return isset($_GET['preview']) && isset($_GET['p']) && $_GET['p'] != $postID;
    }
}


$posts = new Posts;
add_action('the_posts' , array($posts, 'manipulate'), Posts::CATCH_POST_PRIORITY);