Making sure mod_expires is enabled
sudo a2enmod expires
systemctl restart apache2
Add Expires Headers to .htaccess
To add Expires Headers to your site, you need to edit the .htaccess file.
Simply download the .htaccess file from the root of your host (it may be hidden) and add the code below:
## EXPIRES CACHING ## <IfModule mod_expires.c> ExpiresActive On ExpiresByType image/jpg "access plus 1 month" ExpiresByType image/jpeg "access plus 1 month" ExpiresByType image/gif "access plus 1 month" ExpiresByType image/png "access plus 1 month" ExpiresByType text/css "access plus 1 month" ExpiresByType application/pdf "access plus 1 month" ExpiresByType text/x-javascript "access plus 1 month" ExpiresByType application/x-shockwave-flash "access plus 1 month" ExpiresByType image/x-icon "access plus 1 year" ExpiresDefault "access plus 2 days" </IfModule> ## EXPIRES CACHING ##
References
https://electrictoolbox.com/apache-mod-expires-browser-caching/
https://betterstudio.com/blog/add-expires-headers-htaccess/