Metadata
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DITA has a solid vocabulary for specifying metadata. There are <prolog> elements in the topics, and <topicmeta>, <bookmeta> elements in the bookmaps. They can be used to define authors, dates, audiences, organizations, etc. See: https://www.oxygenxml.com/dita/1.3/specs/archSpec/base/metadata-in-maps-and-topics.html

It is up to you to decide where this information should be presented, in the PDF content or in the PDF document properties.

Metadata - Input XML fragment
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In the merged map, the metadata section is placed inside the <oxy:front-page> element. This is different from the original placement in the map or bookmap - after the title, but allows usage of information from it in the title page.

Bookmaps

This is an example of a section taken from a merged bookmap. It only contains some of the possible metadata elements. The bookmeta metadata section is inherited from topicmeta:

<bookmap xmlns:ditaarch="http://dita.oasis-open.org/architecture/2005/"
  xmlns:opentopic-index="http://www.idiominc.com/opentopic/index" cascade="merge" 
    class="- map/map bookmap/bookmap "
  ditaarch:DITAArchVersion="1.3" >
    
  <oxy:front-page xmlns:oxy="http://www.oxygenxml.com/extensions/author">
       
   <bookmeta xmlns:dita-ot="http://dita-ot.sourceforge.net/ns/201007/dita-ot"
     class="- map/topicmeta bookmap/bookmeta ">
     <author class="- topic/author ">Howe Tuduit</author>
     <bookid class="- topic/data bookmap/bookid ">
       <isbn class="- topic/data bookmap/isbn ">071271271X</isbn>
       <booknumber class="- topic/data bookmap/booknumber ">SG99-9999-00</booknumber>
        <maintainer class="- topic/data bookmap/maintainer ">
        <organization class="- topic/data bookmap/organization ">ACME Tools</organization>
        <person class="- topic/data bookmap/person "/>
       </maintainer>
     </bookid>
     <bookrights class="- topic/data bookmap/bookrights ">
        ...
         <bookowner class="- topic/data bookmap/bookowner ">
  <organization class="- topic/data bookmap/organization ">ACME Tools, Inc.</organization>
         </bookowner>
     </bookrights>
        </bookmeta>
        
        <oxy:front-page-title>
            ...
...

For the DITA Map PDF - based on HTML5 & CSS transformation type, the merged map is further processed resulting in a collection of HTML5 <div> elements. These elements preserve the original DITA @class attribute values and add a new value derived from the DITA element name.

<div 
    class="- map/map bookmap/bookmap bookmap" ... >
    
   <div class=" front-page/front-page front-page">
       
   <div 
     class="- map/topicmeta bookmap/bookmeta boometa">
     <div class="- topic/author author">Howe Tuduit</div>
     <div class="- topic/data bookmap/bookid bookid">
       <div class="- topic/data bookmap/isbn isbn">071271271X</div>
       <div class="- topic/data bookmap/booknumber booknumber">SG99-9999-00</div>
       <div class="- topic/data bookmap/maintainer maintainer">
          <div class="- topic/data bookmap/organization organization">ACME Tools</div>
          <div class="- topic/data bookmap/person person"/>
        </div>
      </div>
      <div class="- topic/data bookmap/bookrights bookrights">
        ...
         <div class="- topic/data bookmap/bookowner bookowner">
              <div class="- topic/data bookmap/organization organization">
                                          ACME Tools, Inc.
              </div>
         </div>
       </div>
     </div>
        
     <div class=" front-page/front-page-title front-page-title">
            ...
...

Maps

The maps have a more simple structure, they use the <topicmeta> element for metadata sections. This is also a simplified example, as there may be many more elements in the metadata section:

<map xmlns:ditaarch="http://dita.oasis-open.org/architecture/2005/"
    xmlns:opentopic-index="http://www.idiominc.com/opentopic/index" 
          cascade="merge" class="- map/map "
    ditaarch:DITAArchVersion="1.3">
    ...
    
<oxy:front-page xmlns:oxy="http://www.oxygenxml.com/extensions/author">
    
  <topicmeta class="- map/topicmeta ">
      <author class="- topic/author ">Dan C</author>
      <metadata class="- topic/metadata ">
         <prodinfo class="- topic/prodinfo ">
             <prodname class="- topic/prodname ">oXygen PDF CSS DITA Plugin</prodname>
         </prodinfo>
      </metadata>
      <audience class="- topic/audience "/>
  </topicmeta>
...

For the DITA Map PDF - based on HTML5 & CSS transformation type, the merged map is further processed resulting in a collection of HTML5 <div> elements:

<div xmlns:ditaarch="http://dita.oasis-open.org/architecture/2005/"
    xmlns:opentopic-index="http://www.idiominc.com/opentopic/index" 
          cascade="merge" class="- map/map "
    ditaarch:DITAArchVersion="1.3">
    ...
    
<div class=" front-page/front-page front-page">
    
  <div class="- map/topicmeta topicmeta">
      <div class="- topic/author author">Dan C</div>
      <div class="- topic/metadata metadata">
         <div class="- topic/prodinfo prodinfo">
             <div class="- topic/prodname prodname">oXygen PDF CSS DITA Plugin</div>
         </div>
      </div>
      <div class="- topic/audience audience"/>
  </topicmeta>
...

Metadata - Built-in CSS rules
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The [PLUGIN_DIR]/css/print/p-meta.css file contains the rules that extract metadata.

How to Add the Publication Audience to the Custom PDF Metadata
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The audience element indicates the users the publication is addressing. This can be placed inside a <topicmeta> element in a <map> as in the following example:
<map>
    ...
    <topicmeta>
        ...
        <audience type="programmer" job="programming" experiencelevel="expert"/>

To collect the @type attribute, add the following in your customization CSS:

*[class ~= "map/map"] > *[class ~= "map/topicmeta"] > *[class ~= "topic/audience"] {
    -oxy-pdf-meta-custom: "Audience" attr(type);
}
Note: It is best to use the class selector (such as *[class ~= "map/topicmeta"]) instead of topicmeta to cover cases where the elements are specialized (for instance, in a bookmap the bookmeta is a topicmeta, so your selector will also function for bookmaps, not only simple maps.
Note: The selector begins with map > to choose the <topicmeta> that is a direct child of the map, not other <topicmeta> elements from other <topicref> elements.

The metadata is displayed in the Custom tab of the Document Properties dialog box from Acrobat Reader:

Document Properties dialog in Acrobat Reader

How to Show Metadata in the Cover Page
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Suppose you need to present the author and the ISBN just under the publication title. Your bookmap contains:
<bookmap id="taskbook">
    <booktitle>
        <booklibrary>Retro Tools</booklibrary>
        <mainbooktitle>Product tasks</mainbooktitle>
        <booktitlealt>Tasks and what they can do</booktitlealt>
    </booktitle>
    <bookmeta>        
        <author>Howe Tuduit</author>
        <critdates>
            <created date="1/1/2015"/>
            <revised modified="3/4/2016"/>
            <revised modified="3/5/2016"/>
        </critdates>
        <bookid>
            <isbn>071271271X</isbn>
            <booknumber>SG99-9999-00</booknumber>
...

The entire <booktitle> element content is displayed on the first page of the PDF, so suppose you need to add the information after it.

In your customization CSS, add the following CSS rules:
*[class ~= "bookmap/booktitle"]:after {
    display:block;
    content: "by " oxy_xpath('//*[contains(@class, " bookmap/bookmeta ")]/*[contains(@class, " topic/author ")]/text()');
    margin-top: 4em;
    text-align: center;
    color: gray;
}
*[class ~= "bookmap/booktitle"]:after(2) {
    display:block;
    content: "ISBN "oxy_xpath('//*[contains(@class, " bookmap/isbn ")]/text()');
    text-align: center;
    color: gray;
}
The following CSS extensions were used in this example:
  • oxy_xpath - Executes an XPath expression and returns a string content. Use this whenever you need to extract data from elements other than the one matched by the CSS rule selector.
  • :after(N) - Creates more than one after pseudo element. The argument value represents how far the generated content is from the real content. In the example above, the content of the :after is closer to the title (upper) than the content of the :after(2).
Note: The attr() CSS function can also be used but is limited to extracting attribute values from the matched element.

Generating Synthetic Pages for Metadata.

Suppose you need to show this information on a page that follows the title page, instead of on the title page. In this case, you need to prepare a named page and place the content on it. In your customization CSS, add the following CSS rules:

@page page-for-meta {
    background-color: yellow; /* Just to see it better*/
    @top-left-corner {
        content:""; /* Remove the default header */
    }
    @top-right-corner {
        content:""; /* Remove the default header */
    }
}

*[class ~= "bookmap/booktitle"]:after {
    page: page-for-meta;
}
*[class ~= "bookmap/booktitle"]:after(2) {
    page: page-for-meta;
}