There is NO such document. There is however "official secrets act" which prevents people from leaking out state secrets and goes for high ranking ex army officers as well politicians. You can google it.
^There does exist an army act of 1952 which was amended during Musharraf term. I don't beleive that its a secret act, i think it is a public document but i have not been able to find an online copy, may be some lawyers will know better.
You guys are confusing two difference documents:
1) Official Secret Act is in the Constitution of Pakistan only
2) Army Act is in Constitution of Pakistan only
3) Army Constitution is totally independent document. It provide chief with powers to take any step for the sake of nation. Including overthrowing political government or Passing Emergency Laws or Change in the Constitution of Nation. Chief powers can't be challenge in court or in Parliament, orders authorized by Chief can only be repeal by Army Chief.