digicert public certificate On iPad

Without understanding your explicit requirement - or whether your iPad is centrally managed by a school or Enterprise - a definitive answer is impossible to provide.

For the most part, for privately owner devices, it shouldn’t be necessary to add a Root Certificate to your trusted Certificates. If you don’t understand the implications of changing/adding PKI Certificates to your device, then you definitely shouldn’t be exploring such changes. You should instead seek advice and guidance from your System Administrator.

These resources might be helpful to you:

Lists of available trusted root certificates in iOS – Apple Support

Trust manually installed certificate profiles in iOS and iPadOS – Apple Support