Are PDF documents that are uploaded into Vuture secure? and if so how?"
Yes - data at rest in Vuture's production environment is encrypted, with access to production highly restricted and monitored.
However, anyone with the link to an uploaded PDF can download the PDF (if they are linked to from an email and someone posts that email online then the link to the PDF could become more widely available) unless you use the Secure Page feature - https://support.vutu.re/hc/en-us/articles/4409623503121-Securing-a-Page-in-Vuture-Making-Pages-Available-to-Logged-In-Users-Only or automated access.

When Secure Site Pages is enabled, how do you then assign it to the container?
You don't assign it to the container. It is on a per page/file basis.

 

To confirm, contacts would need to be able to log into Vuture first to see the document?

Yes contacts would need to be logged in to Vuture.

I've checked the help site and I can see this - https://support.vutu.re/hc/en-us/articles/360020702098-Lock-Down-Front-End-Access-To-a-Container-By-IP-Address

How would this be set-up and would this then lock the document down by IP address?
To lock a container by IP address, raise a support ticket with the required IP addresses and containers.

 

If a client enabled this for a container, will it retrospectively go back and mean people can't access files if they've already received the email?
Yes.  But if the contacts have already downloaded it and saved it locally then of course it won't stop that.