Possible to make website only accessible by those with links or only people emailed?

I've made a website which allows people to download a custom pdf certificate. The certificate is kind of an exclusive thing so I don't want just anybody to be able to view the site and get a certificate. The way people would get to the site normally is that I'd email a link to the people. Is it possible to make it so that only people emailed the link could view the site? Thanks.