How to get the permanent link in a plugin?

globalize $post to get the current post’s data. also, you want get_permalink, which returns the permalink, rather than the_permalink, which directly echoes the permalink.

function abcd_add_contents($content) {

    if (is_single()) {
        global $post;
        $extra_content = get_permalink( $post->ID );
        $content .= $extra_content;
    }
    return $content;
}

add_filter('the_content', 'abcd_add_contents');