"Supposed to" has the same meaning as "should", but it's more intense.
I should go there.
I'm supposed to go there.
Both are very similar, although "supposed to" is a bit stronger, like someone told you to go there and you are expected to come, therefore you are "supposed to go".
Sometimes you hear a phrase
This was not supposed to happen.
Again, it's very similar to "this shouldn't have happened", although again, a bit stronger, almost like a destiny, fate.
Supposed to is formally correct, so yes, you can use it in an official document, as long as it's not misused. So how could it be misused, you might ask?
Misused example (wrong):
I'm not supposed to stay in your company anymore.
Obviously wrong. Unless you figured out a formula for calculating the future, you cannot make such a statement.
Proper example:
The accident happened because Peter, who was supposed to take care of the overheating problem, forgot to complete his task.
Perfectly proper. Peter's boss ordered him to take care of the overheating problem and Peter forgot. He was supposed to do it, but he didn't, because he forgot. You can make such a statement.