DTD in PDF?

The question arose in discussion of the ACXO, does the present spec allow
one to describe a DTD that is presented in PDF?  The answer is yes

from data-element.dtd:

<!ELEMENT representation (
        representation-label,
        datatype-label?,
        format?,
        character-set-name?,
        unit-of-quantity? )>

<!ELEMENT representation-label      (#PCDATA)>
<!ATTLIST representation-label
        xsgml-type (%xsgml-entity-list;) #IMPLIED
>
...
<!ELEMENT format             EMPTY>
<!ATTLIST format
        mime-type       (%mime-type-list;) #REQUIRED
>

as used in db1.txt:

<representation>
<representation-label
  xsgml-type="sgml-dtd"/>
</representation>

which is therefore an incomplete example.  I didn't provide a text string as
content of representation-label because I figured it could be autogenerated.
But, as Ron Schuldt pointed out, plain text is only one format for a DTD;
it could also be presented in PDF or HTML.  So I should have added a
format element,

  <format mime-type="text/plain"/> 

for the db1.txt example, and for the same presented in PDF it would be

  <format mime-type="application/PDF"/> 

I append the relevant documentation (from rrdtddoc.hrm, slightly maimed
by having been cut-and-pasted from HTML):

representation 
Part 1, definition 3.3.60: “the combination of a value domain, datatype, and, if 
necessary, a unit of measure or a character set.” Part 1, definition 3.3.61:: “type 
of symbol, character or other designation used to represent a data element.” For 
example, a DTD may be supplied in text or PDF. ISO/IEC 11179 supplies more 
content for this element than seems needed for registering DTDs and schemas,
 but it has been retained anyway: a label, a label for a datatype, a description 
of format, a name for character set, and a unit of quantity. 

representation-label 
in ISO/IEC 11179, “representation term”, here renamed for consistency and 
employed a bit loosely; Part 1, definition 3.3.62: “a component of a data element 
name which describes the form of representation of the data element.” Its 
content is a string and the value of its xsgml-type attribute is drawn from 
xsgml-entity-list.ent. This element indicates whether a registered entity is 
a DTD, a schema, general text, and so forth. 

datatype-label 
a string indicating the datatype of the data element or data element 
dictionary; possibly overlapping with representation-label. Part 1, 
definition 3.3.25, for “datatype”: “the format used for the collection of 
letters, digits, and/or symbols, to depict values of a data element, 
determined by the operations that may be performed on the 
data element.” 

format 
an empty element, indicating through its mime-type attribute the MIME 

type of the registered entity. 

character-set-name 
a string naming the character set of a registered entity. 

unit-of-quantity 
a string naming the unit of quantity used for values of a data element;
 not relevant to registration of DTDs and schemas. 

regards, Terry