Q: How can I get the acceptance letter for a paper which I have co-authored?

Your question is not very clear. I suppose when you say you are a “coworker,” you mean you are a coauthor of the paper. Also, I think you have used the term “responsible writer” to refer to the corresponding author. Journal editors exchange emails only with the corresponding author, not with the co-authors. The corresponding author is the single point of contact for the journal. It is the duty of the corresponding author to communicate the editor’s messages to the co-authors. It is not possible for journal editors to send separate acceptance letters to each and every co-author. Therefore, I think you should contact the corresponding author and ask him/her to forward the acceptance letter to you if you need it. I don’t think it is advisable to write to the editor for this.