The standard Media Type (formerly known as MIME types) is
application/pdf. The assignment is defined in RFC 3778, The application/pdf Media Type, referenced from the Media Types registry.
Media 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 Media Type for PDF. Media 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.