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.
The use of
x-pdf predates the standardization of the MIME type for PDF. MIME types in the
x- namespace are considered experimental, just as those in the
vnd. namespace are considered vendor-specific.
x-pdf might be used for compatibility with old software.