The PDF export template has a new field named "Metadata" that you can use to display additional information about the exported wiki pages in the PDF header or footer. You need to follow three steps:

• Use the Metadata field to indicate the extra information you want to display. For instance, if you want to display the tags:
  {{velocity output="false"}}
  $metadata.putAll({
    'data-tags': $stringtool.join($doc.getTagList(), ', ')
  })
  {{/velocity}}
• Use the Header or Footer fields to indicate where to display the metadata:
  {{html clean="false"}}
  Tags: <span class="pdf-doc-tags"></span>
  {{/html}}
• Use the Style Sheet Extension object to inject the metadata in the PDF header or footer:
  h1[data-xwiki-document-reference] {
   string-set: doc-tags attr(data-tags);
  }
  .pdf-doc-tags:before {
   content: string(doc-tags);
  }

See the PDF Export Application documentation for more information.