Sharepoint search Documents from subsites

Hi,

I have site collection, with 10+ sub sites. each of the sub site have document library which contains some documents with metadata.

Now, I want to implement the functionality  where I can able to search all these documents at site collection level, filter and group by meta data.

I can create Master Document library at site collection level and Push documents from each site to Master library with workflow, but that will create another copy of document.