To add a sense of reality to fiction, authors sometimes use brand named products, but should they?
The short answer is: It depends.
We've all found brand names in books, TV shows, and movies. In fact, many viewers become annoyed with obvious branding in movies, so studios have become more circumvent in their product placement. Of course these are paid placements, so the company in question knows how their products are represented.
Since you're writing fiction, it's pretty clear that you aren't trying to make a competing product with any company, so there is no trademark infringement. However, you might be risking defamation charges if they think their products are presented in a negative way. If your character is adamant about not liking a certain brand of fried chicken, that company can say you were defaming their name if they can prove readers thought your accusations were presented as a fact.
Even if you don't say anything bad about the products themselves, your story or characters may not present the company in a way they like, so you risk charges of tarnishing the company name. You can obtain permission if you think it's critical to your story, but that might be costly and time consuming.
If that brand is critical to your story, you have turned it into a character, and it would be best to make up your own version to prevent potential law suits. Kathryn Goldman has a wonderful article describing some of the considerations for branding used in The Fault in Our Stars by John Green.
So, should you write about your primary character drinking a Pepsi while driving his Ford F-150 to McDonald's?
There is nothing saying you can't, but you do want to be very careful how you do it.
Our society is so brand heavy that we all have preconceived notions about specific brands base on our experiences. Just because you love Pepsi, doesn't mean your readers do. And if they absolutely hate it for some reason, you've alienated your readership. Read more about that in Tamela Hancock Murray's post.
Another issue is cramming your book so full of details that your story gets lost. We all tend to gloss over details we don't think are important, and you want your reader to stay in the story. K.M. Weiland wrote a great example of over branding here.
Ask yourself if those specific products make a difference to your story. If the same character above could be driving a pick-up truck and drinking a cola while he heads to the burger shop, you might want to skip the brands. If your reader gets a positive mental image by inserting their favorite brands, and doesn't hurt your story, everyone is happy.