What do you mean by combine? PCC is a load balancing set up, not a bonding solution, this means that a given connection will only be able to consume the bandwidth available on the line it is going out of. Bonding, in most cases, is not possible to do on the WAN side of things unless you have multiple connections from the same ISP and they are willing to set up bonding for you, or you have another device at some other location with more bandwidth to bond to.
As for the distinction between web browsing and downloading, as far as the router is concerned and knows, they are one and the same thing. An HTTP page download and an HTTP download for an ISO file are exactly the same thing, so there is no real way to make a distinction between them. You can classify stuff based off of the connection bytes that have been transferred to make a distinction, but that is as close as you are going to get to "seeing" a difference between a client web browsing and downloading a file via HTTP. Also as a note, the router has no way of knowing how much data will be transferred with any given connection, it only knows that after a connection has been closed.