Hello. I am not a specialist of English, but I am pretty sure you shouldd say
I'm going to do it after I get my documents. (the actions go after each other)

I'm going to do it before I get my documents.  (it is pretty much the same thing)

You should use present perfect when you want say that an action happened at an unspecified time before now. For example: I haven't been there before.

Now look at your senteces. You are talking about future, not about present time or so to say ''now''. You cannot use present perfect. :)

I hope it helps :)

Could you write a correction please?

I can't understant the difference between perfect tenses. For example:

I'm going to do it after I've got my documents. This is correct. "After" indicates that an action was in the past.

or

I'm going to do it before/once I've get my documents. This is correct too!  "Before" means that the event of getting the documents has not happened yet.

"I shall/will do it when I receive the documents" is another way of saying.  However, all these forms have a slightly different intent.  In the end, it conveys the same sentiment. A is to be done when B happens.

Which of examples is correct? or both are incorrect?