Anyone with the link can view your credential, but some options only appear for you, the owner, once you are logged in. These include Download PDF and Add to LinkedIn Profile.
If these options are missing
Log in. Go to app.certify.one, or open the link in your credential email. If you haven't set a password yet, use the forgotten password option to create one.
Use the right email address. You need to be logged in with the email address the credential was issued to. If you are logged in with a different address, you will see the credential as a visitor would, without the owner options.
Why the email address matters
Certify credentials follow the Open Badges standard, which uses your email address to identify you as the owner. A credential stays linked to the address it was issued to. If you would like it moved to a different address, ask the organisation that issued it. See When to contact your issuer.