Add signature to post/pages with the_content filter

functions.php

add_filter( 'the_content', 'add_my_signature' );
function add_my_signature( $content ){

	if( ! is_single() ) // if were not on a post/page just return the content.
		return $content;

	$sig = '<br />Thanks for reading!'; // Add a sig to the post/page content and return.
	return $content . $sig;
}