Preview archive

functions.php

<?php

add_action( 'pre_get_posts', 'fpl_maybe_alter_main_query' );
function fpl_maybe_alter_main_query( $query ) {
	
	//only perform this action on the main query on the frontend
    if ( $query->is_main_query() && ( ! is_admin() ) ) {
	
		//only do this for logged in users
		if ( is_user_logged_in() ) {
			
			//only do this when a preview is being requested
			if ( isset( $_GET['preview'] ) && ( $_GET['preview'] == true ) ) {
				$query->set( 'post_status', 'any' );
			}			
		}		
    }	
}