Yeah, I think that's pretty much the case. The first 320 bytes of the two PDFs released by Google result in the same SHA-1 state. Once you're at that point as long as you append identical data to each of the files you're going to get identical hashes. This is just taking those same 320 bytes and appending the combined images of your choice.
edit: as versteegen points out it's 320 bytes, not 304.
edit: as versteegen points out it's 320 bytes, not 304.