Prabhu,
You can create workflow for document approval and then link it to 1st state of lifecycle (u need to use advance lifecycle ),so whenever new document is created/uploaded workflow process will start. Create workflow as per your approval process i.e. add approve review and rework activities.
Check Windchill l help centre for more details
else you can define necessary transition in Document lifecycle for promote & lock transition and use promotion request for document approval
check help Centre for more details
http://support.ptc.com/cs/help/windchill_hc/wc102_hc/index.jspx?id=LCStateTransDefine&action=show
Hope it helps
Thanks
Shreyas