[Ray76]

Perhaps because it was not printed on security paper. The "security paper" is part of the image.

Something like this can get you started:
http://ep.yimg.com/ca/I/yhst-37067711489792_2260_5312874

[4Zoltan]

The image you link is just the green background. The pdf has more than just a green basketweave pattern. It has graphics (lines) and text.

Take for example box 6c on the birth certificate. It says “6c Name of Hospital or Institution (if not in hospital or institution, give address)” but when you turnoff the text layer, the green background says “6c N f H al I (if h tal add “. Why would a forger put some of the letters on the background and some on the text layer?