If you don't reference the resource in the DITA map, then it won't be included in the output. Add the following to your map, marking the resource as processing-role="resource-only".
<topicref href="nondita_resources/docmap.pdf" format="pdf" processing-role="resource-only"/>
Now that you've ensured the resource will be included with the output files, use an xref and point to the resource:
<xref format="pdf" href="nondita_resources/docmap.pdf" scope="peer">sample pdf</xref>
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Set the format. You can type any value you want there, not just those listed in the value choices. Also add the scope attribute: scope="peer".
