I want to talk to my biological mother only to get some documents I want
Do you want to talk to her for other reasons?
No, I just want a few of her letters and some documents I need.
I'm sorry you are feeling that way. It's understandable that sometimes people don't have a close relationship with family members. You could always reconcile or just let her know that you are not ready to talk just yet.
It's fine I can do whatever I want.
I think that being honest and telling her that you only need the documents sets a boundary and you will be thankful for it.