Can we add document as subassembly in a part ? is it possible ?
Sort of. Just make a light-type of WTPart with its own icon, name and rules (i.e. "Structure Document" or "Docupart" or whatever makes sense to you), and then link a reference or described by document (depending on how you defined the document and you intent for revising the document with or separately from the WTPart) so it will show up in the structure with a simple "Show Documents."
You can even get crazy and make a simple java service to listen for the creation of this type of Document (because you make a similar light-type of document) that will auto-generate a already-linked WTPart of the appropriate type and attribute values. We do this already with Operations in MPMLink 10.1 where Operations cannot have Assigned Choices, but Windchill Parts in a bill of material can. Therefore, we have a service that listens for the creation of an Operation at which time we automatically create an equivalent WTPart that we automatically link to the next higher part number that is linked to the Process Plan that it the next higher item for that Operation.
You cannot simply put a document in as a part, however, because a document is not a part.
Could you explain the reason behind adding document as an part to assembly?
Why do not just add a Document as reference link to the WTPart ?
It behave exactly like the structure link, and is visible in Product Structure Explorer
You can also create "Notes" in BOM. But note sure you are able to add some doc content inside ....
If the idea is to do as for SAP erp , with BOM items of type "Doc" , you will need to customize.