How to add content at the end of posts?

You can simply use get_field that returns the field instead of the_field that echo it.

function custom_content_filter_the_content( $content ) {
    if ( function_exists('get_field') ) {
      $content .= '<span class="custom_fds">' . get_field('field1') . '</span>';
    }
    return $content;
}
add_filter( 'the_content', 'custom_content_filter_the_content' );