That's because it's a 19 MB JPG with 24,000x18,000 pixels.
How about some common sense ... I can download it, and get to see a bit of it, but very difficult to see anything. It would be a lot more funcational if you hadn't overlain overtop of a photograph ...
I knew it may not display properly in the forum thread itself, but I figured opening it in a separate window would be ok (it worked on mine). I've uploaded a new version that doesn't have the image background. Sorry for the browser crashes!
Without background: https://dl.dropboxusercontent.com/u/43869799/GTA System Map - To Scale_NoBackground.jpg
Full PDF version (big map, be warned): https://dl.dropboxusercontent.com/u/43869799/GTA System Map - To Scale.pdf