A Multi-WAN router is the most cost effective way if the time per file is ok but your current isp line is clogged.
As I understand it, with a multi-WAN router if you have ten 100MB pdf files waiting to transfer and you add a multi-wan router and another isp line, ideally half will go over one line and half over the other (or approximately half.)
However if you have one huge 1GB pdf of all the pages together in a single file you're waiting to transfer, a multi-wan router alone won't provide faster transfer of that file except having the second connection for other files as the single file will still go over one of the two lines (unless it can be broken up into smaller pieces.)
Also a mutl-wan router alone won't work if your transfer is over a secure connection that can't tolerate the source or destination being multiple lines at once.
Ideally your ISP would provide a bonding service, but only some do.
Another third-party bonding service to compare with mushroom networks is now shared band -
www.sharedband.com
It supports either two or four bonded ISP connections and the hardware is more affordable than mushroom's with a choice of their "power routers" or modified firmware off-the-self routers that work in conjunction with their POPs to provide bonded lines that appear to the outside world to be a single "normal" isp connection. They also have more POP locations in the US than Mushroom Networks meaning they can potentially provide a POP closer to you for less added latency.