If you're subscribed to a stream then all offchain items for that stream will be automatically downloaded, as soon as the item is seen, and locally stored (this behavior can optionally be changed in MultiChain Enterprise only). The same applies if the subscription is created by autosubscribe.
The downloading is fast but asynchronous – see the getchunkqueueinfo command.
Downstream subscribers will also maintain redundant copies and deliver them to other nodes in the network requesting them, so the original issuer doesn't (in theory) have to maintain their copy perpetually. But you will need to use MultiChain Enterprise (and the purgepublisheditems command) in order to remove the local copy from the original publishing node. And of course this should be done with serious caution!
