If I do get an offer (assume the offer is presented to me in person),
is it okay for me to tell HR that I am going to require 5 days before
making a decision?
Five days is on the long side, but it's not unreasonable. I've had folks take a week or so before deciding when I was the hiring manager. For me, that was the limit, but every situation is different.
It would be unusual to state "5 days" specifically.
You may be better off phrasing it as "Thank you! I'll need a few days to look this over and think it through before I get back to you. Is that okay?"
That removes the awkward "5 days" aspect.
If pressed for "when can we expect your answer?" you could say something like "I'll get back to you next week" and see if that is acceptable. And if not, you can promise to get back to them sooner.
(Note that it would be a red flag for me if a prospective employer wanted an immediate answer after their initial offer. That's usually a bad sign.)
I just came out of university so HR knows I'm not currently working. I
feel like if I don't provide a reason as to why I need 5 days, then it
will seem weird. Is it okay for me to tell them that I am expecting
other offers soon so I think it is best for me to wait 5 days before
It normally won't seem weird to ask for some time to think about it. After all, this is a big career-launching decision. For me, it would be weird if you didn't take a few days to think about it.
I wouldn't offer a reason, unless asked.
When I was contemplating an offer, I always took at least a few days, even if I knew in my heart I liked the offer. I always wanted to think through the details, and talk it over with others whose opinions I valued.
If you feel strongly about it, you could disclose that you are expecting other offers. But don't push the envelope too much here. Do get back to this potential employer in a reasonable time period (in my mind, at least within a week), or you risk having them move on to another candidate. At some point, an employer will assume you don't want the job and withdraw the offer.