The file mimetype.rdf in the profile folder of zdesktop has to be modified for the extension pdf.
<RDF
escription RDF:about="urn:mimetype:handler:application/pdf"
NC:saveToDisk="false"
NC:alwaysAsk="false"
NC:useSystemDefault="true">
<NC:externalApplication RDF:resource="urn:mimetype:externalApplication:app lication/pdf"/>