A document library that is available in the initial portal installation may also be available in each virtual portal. This depends on personalization being available in that virtual portal and being configured appropriately concerning that document library.
Searching for a document in a document library will produce a document reference (URL) that is different in each virtual portal, but these discrete references point to the same document in the document library. To provide separation of content within virtual portals, use separate document libraries for each virtual portal. To provide content collaboration between virtual portals, use the same document libraries between virtual portals.