Proper MIME type for PDF files

When working with PDFs, I've run across the MIME types 'application/pdf' and 'application/x-pdf' among others.

Is there a difference between these two types, and if so what is it? Is one preferred over the other?

I'm working on a web-app which must deliver huge amounts of PDFs and I want to do it the correct way, if there is one.

From stackoverflow

• The standard MIME type is application/pdf. The assignment is defined in RFC 3778, The application/pdf Media Type, referenced from the MIME Media Types registry.

  MIME types are controlled by a standards body, The Internet Assigned Numbers Authority (IANA). This is the same organization that manages the root name servers and the IP address space.

  Kip : so... why does x-pdf get used sometimes?