Open book exam tomorrow but the file with the questions are open to everyone already
Tell your professor.  I don't know whether Canvas keeps logs such as you describe, but it doesn't make any difference.  Your feeling of moral obligation is correct. You are obligated to tell your professor.
Here is some general advice.  Author John D. MacDonald has one of his characters say, approximately, &quot;In the case of any ethical decision, the thing you least want to do is probably the right choice.&quot;  (Quoted from memory, which is why it's approximate.  I think Benedict De Spinoza wrote something similar.)