> But Google would be able to produce two PDF files that would, as git sees them, appear to be same just as easy as these that were produced.

Right, but they would have to re-do their enormous calculation. ("This attack required over 9,223,372,036,854,775,808 SHA1 computations.")

Google started with a common prefix p (the PDF header), then computed blocks M11, M12, M21 and M22, such that (p || M11 || M21 || S) and (p || M12 || M22 || S) collide for any suffix S. Given p, M11, M12, M21 and M22, anyone can make colliding PDFs that show different contents quickly. But to generate a collision with a different prefix q, e.g. one including the file size, one would have to do the expensive computation all over again, I think.

Note: I'm not trying to argue that SHA-1 can be made secure with padding. I was just trying to say that the statement "The PDFs have the same size" misses the point.