How to export a Virtual Document in Documentum.

First you will need to convert the IDfSysObject to an IDfVirtualDocument.

        //verify the object is a virtual document
 //the variable doc is the IDfSysObject that you are working with
        if (doc.isVirtualDocument()) {
           IDfVirtualDocument v = doc.asVirtualDocument("CURRENT",false);
           exportVirtualDocument(v, exportDir, "");
       } else {
           throw new DfException("Must be a virtual document to export.");
       }

Then take the doc variable and call this method:

    /**
   * Exports virtual document to the destination directory
   *
   * @param doc virtual document to be exported
   * @param destDir directory virtual document should be exported to
   * @param exportFormat export format (such as html, or you can use the da to create a new one)
   * @throws Exception
   */
  private void exportVirtualDocument(IDfVirtualDocument doc,
          String destDir, String exportFormat) throws Exception {
      IDfClientX clientx = new DfClientX();

      IDfExportOperation operation = clientx.getExportOperation();
      operation.setDestinationDirectory(destDir);
      IDfExportNode node = (IDfExportNode) operation.add(doc);
      node.setFormat(exportFormat);

      // Execute the operation
      boolean executeFlag = operation.execute();

      // Check if any errors occured during the execution of the operation
      if (executeFlag == false) {
          // Get the list of errors
          IDfList errorList = operation.getErrors();
          String message = "";
          IDfOperationError error = null;

          // Iterate through the errors and concatenate the error messages
          for (int i = 0; i < errorList.getCount(); i++) {
              error = (IDfOperationError) errorList.get(i);
              message += error.getMessage();
          }

          if (message.length() > 0) {
              // Create a DfException to report the errors
              DfException e = new DfException();
              e.setMessage(message);
              throw e;
          }
      }
  }