I don't necessarily mean a bad smell like cheese for example, even oranges or a hot soup. Especially if it's in an area where customers pass by. (might seem unprofessional)
Is eating smelly things in the office rude?
You should assume "yes" because by doing this you are disregarding the impact that this has on people that cannot control it and must take action to try to handle it, or you are unaware of the problem it creates for them. Either way, it should be avoided.
The word "rude" means "not showing good manners toward others". Good manners are a way of helping the people around you to be comfortable and acting in such a way that you demonstrate awareness of others. So, by eating anything that has an odor that carries beyond your immediate vicinity in an office, unless you are keenly aware that everyone in smelling distance will enjoy it (or, at a minimum, not be bothered by it), you are being discourteous/rude.
So proceed with caution, since some smells in some offices are "acceptable" while most should simply not be brought into the workplace.
EDIT: The question was changed to:
Is eating smelly things in the office unprofessional?
Again, the answer is "yes" in general. Being a professional means being aware of how your behavior impacts those around you. Professional manners are every bit as important as social manners, especially because people are brought together out of necessity and are usually required to spend far more time in close proximity compared to social situations.
The approach to foods that smell is very similar to music, appropriate lighting, political/religious discussions or any other situation where the activity may cause distraction or offense. With smell (and maybe music for some people) even nausea or physical illness.
This is highly workplace dependent. You're right that even a delicious smell might distract someone and make them wish they had an orange or whatever you're having.
Some workplaces have a lunch room and expect you to go there to eat. This is not just so that your lunch (smells, eating sounds, possible mess) doesn't affect your coworkers, but also so that you'll take a break and maybe chat with others. This generally serves the company's goals. Other workplaces expect you to leave the building and buy lunch, and consider bringing your own a little odd, or eating a snack in the morning or afternoon a little odd. Still others expect you to eat at your desk and work as you do so.
What you need to do is learn what the norm is for your workplace. If people sometimes go to the lunch room and sometimes eat at their desk, observe the pattern to see why they choose one or the other. Maybe on a busy day they eat at the desk, but maybe it's more a matter of what they're eating that day. If anyone comments on your food choices, whether joking that "oh, you're making me so hungry I can't wait for my lunch!" or more seriously saying "you know a customer is coming this afternoon, don't you?" pay very close attention. They are telling you the norms for your workplace.
You may find that the solution is to change what food you bring, rather than where you eat it. That is, your sardine sandwiches may never be ok, whether in the lunch room or at your desk. Your afternoon orange may be too distracting. Many people eat relatively boring food at the office for just this reason. You can enjoy the more vibrant things at home or if you go out for a meal or a break.
Mileage will vary hugely on this one - it's a mix of cultural norms, nature of the work, sanitary conditions and probably even office air flow that will impact this.
- If you have a break room, use it to eat your food until you observe otherwise
- Some foods have pervasive and strong smells, for these, it's good to have a plan for how to neutralize the smell before you eat it. If it really can't be neutralized and many people find it noxious - then you may want to skip it.
- Allergies usually trump preferences. Be aware if anyone has mentioned an allergy to something airborne or transmitted by contact.
- Office hygiene rules play a factor - obey the rules.
- Office hours usually play a factor - for example, if the office gives a lunch break, it expects you to use it to get food. If the office is having a late night emergency that lasted through dinner, then it's not unreasonable to order some delivered food.
- Be aware of garbage - food trash in a break room is to be expected. Stinky food trash in your cube's trash can all afternoon will probably draw complaints, even when a quick smelly dish does not.
With that said, it does fit into a questionable area. What food stinks, how offensive the smell is, and how horrible it is is both biologically variable (I, for example, have a horrible sense of smell), and culturally variable. In an international workplace, this can make one person's common, everyday food smell awful to someone else. Also - even with a break room, if you end up in a high-demand business, you may end up in a situation where you have no choice but to have food with you, so you can eat in meetings.