Hi Jon,
The content of the index is created by the PlanetPress form design using data selection objects. If you have a client number in the data, create a data selection using that area of your data pane, then in the object's attributes, you will have an "archive/email/fax" tab. Click on it and set the index name.
From now on, every PDF created using that form will have the client number as an index value and will be found in the PDI/xml file.
Create as many data selection objects as you need indexed data selections. You can also use variables and jobinfos from Watch to index in the PDI/xml file.
Regards,
Olivier