<s:download src="/exportPdf.xhtml" rendered="#{myAction.hasPdf}" >
        <h:outputText value="#{myAction.pdfName}"/>
    </s:download>

@Name("myAction")
@Scope(ScopeType.CONVERSATION)
public class MyAction {
private byte[] pdf;
private String pdfName;
public String getPdfName() {
   return pdfName;
}
public void setPdfName(String pdfName) {
   this.pdfName = pdfName;
}
public byte[] getPdf() {
   return dd572pdf;
}

public void setDd572pdf(byte[] dd572pdf) {
   this.dd572pdf = dd572pdf;
}

public boolean hasPdf(){
  return true;
}