There are a number of elements in the example document that only contain PCDATA:
<!ELEMENT category (#PCDATA)>
<!ELEMENT abstract (#PCDATA)>
<!ELEMENT keyword (#PCDATA)>
<!ELEMENT last_updated (#PCDATA)>
<!ELEMENT copyright (#PCDATA)>
<!ELEMENT first_name (#PCDATA)>
<!ELEMENT middle_name (#PCDATA)>
<!ELEMENT last_name (#PCDATA)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT instruments (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT length (#PCDATA)>