The demonstration is a pair of PDFs that display differently. However, Google didn't find two PDFs that just happen to collide. Instead, they built one PDF that

The demonstration is a pair of PDFs that display differently. However, Google didn't find two PDFs that just happen to collide. Instead, they built one PDF that contains two jpegs and a switch that selects one of them for display. The neat bit is that the file has the same hash regardless of the switch's setting. This attack won't work on plain text source because it won't look like source code.