Hi Naga.
I don't think we have this kind of functionalty available where we can export multiple Webi Report in single PDF file.
We can only export multiple tab of same report/document.
This is really usefull functionality...!!!
I would request you to raise this functionality to idea place of SAP Business Object, so SAP will include this functionality in future releases.
Please share the idea link to this thread so we can up vote for it. 
Regards,
Veer