To add far future expires header to you WordPress site simply add this code to the .htaccess file
<IfModule mod_expires.c> ExpiresActive on ExpiresDefault "access plus 1 month" ExpiresByType image/gif "access plus 1 month" ExpiresByType image/png "access plus 1 month" ExpiresByType image/jpg "access plus 1 month" ExpiresByType image/jpeg "access plus 1 month" ExpiresByType text/html "access plus 3 days" ExpiresByType text/xml "access plus 1 seconds" ExpiresByType text/plain "access plus 1 seconds" ExpiresByType application/xml "access plus 1 seconds" ExpiresByType application/rss+xml "access plus 1 seconds" ExpiresByType application/json "access plus 1 seconds" ExpiresByType text/css "access plus 1 week" ExpiresByType text/javascript "access plus 1 week" ExpiresByType application/javascript "access plus 1 week" ExpiresByType application/x-javascript "access plus 1 week" ExpiresByType image/x-ico "access plus 1 year" ExpiresByType image/x-icon "access plus 1 year" ExpiresByType application/pdf "access plus 1 month" <IfModule mod_headers.c> Header unset ETag Header unset Pragma Header unset Last-Modified Header append Cache-Control "public, no-transform, must-revalidate" Header set Last-modified "Tue, 1 Aug 2016 10:10:10 GMT" </IfModule> </IfModule>