I'm a junior hoping to do a PhD. I'm in a graduate level class right now in a different field. I believe almost all the other students are PhD students. The first couple weeks of the course were just barely accessible, though very challenging. Now, however, I really find myself unable to complete the problem sets. There are no office hours either. How bad would it look to pass/fail this class? It's not particularly related to my intended research focus in the PhD. I believe I am a strong candidate aside from this class. Also, I'm really not sure what grade I would get in this class. Probably a B at worst. I'd rather not mess up my GPA though. All of my other technical classes are A's or A minuses.
In short, this looks bad.
When I see a "Pass" or "S" grade on a transcript that I'm evaluating, my default assumption is that the student did work equivalent to the lowest passing letter grade (e.g. a "D" in the US system.)
I tell the undergraduate students that I advise that if they want to go on to graduate school they should avoid taking courses on the S/U grading scale.
No one can predict how an admissions committee or anyone else, such as a potential supervisor, will interpret this, or any particular thing, really.
But, in general, it is the major courses (US system) that most people look at and care about for graduate study. A pass/fail in an upper level major course looks bad. But a pass/fail in an unrelated course, especially an elective, looks much less bad and other things in your application can overwhelm whatever negative aura there might be.
In the case here, dropping the course rather than changing to pass/fail might be another option (or not), and who is to say how a dropped course is viewed.
Elective courses outside the major indicate, to me, at least, that the applicant has a possibly valuable outside interest but didn't want to "risk" GPA. Not a big deal and maybe a positive.