<li>
<apex:commandLink value="Preview Doc" target="_blank" action="{!returnPreviewDoc}">
    <apex:param name="documentId" assignTo="{!documentId}"/>
</apex:commandLink>
   </li>

public PageReference returnPreviewDoc(){
     Document document = [select... where parentid=documentID... limit 1];
     PageReference page = new PageReference('/servlet/servlet.FileDownload?file=' + document.Id);
     return page;
}