Declining an unreasonable request from a superior I've been involved in the publication of a volume, as a collaborator. I did all
the typesetting, among plenty of other things. After the publication, a few
authors asked me to send them a PDF of the final version of their article. I
obliged. Some other authors asked the same thing to the scientific editor of the
volume, who in turn asked me to send her the requested PDF files.

I then learned from a third party that the authors who have received their
article put it online on a well-known academic platform, and that there might be
legal issues about this with the publisher. I told the scientific editor about
this and asked her whether I should create new files with a publisher notice or
something. She answered me to "please tell these researchers to wait a bit
before putting their articles online", so that she can examine the issue.

There's no way I'm going to do that. The scientific editor is a well-known
researcher, and these authors are well-established professors. I'm a PhD student
with zero authority, who got paid for doing the kind of menial/boring work
nobody wants to do.

How should I go about this? I'd tempted to just ignore the request and go on
with my life, but I'm not sure this is the best way to proceed. I don't want
to be in bad terms with the editor of the volume.
Your reluctance is understandable. 

Write to the editor and let him know that while his/her reputation would have standing with these authors, your own would not. It does no one any good if your letter to the authors is sent but ignored. 

Offer to write a letter to them, but for the signature of the editor to be sent officially. The authors are much more likely to wait given a request from the editor. Include anything you think needs to be said, but from the editor's standpoint, not your own. 

An alternative, perhaps nearly as good, is to start out your own letter as "I have been instructed by Dr X, editor, to write to you with the following request and instructions...."