I believe this is a sly reference to the scene in BTTF I where Marty asks for a “Pepsi Free” (a caffeine-free soda) in 1955, or a reference to the similarly alliterative Coca-Cola Classic (the re-named original recipe Coke after the New Coke debacle).
Stylish, isn’t it? Seen in the background are identically-shaped bottles of Slice (Mandarin Orange [opaque metallic red/orange]) and Diet Pepsi (opaque white). Also seen in the background is a bottle of Miller Lite (Marty’s house, IIRC), and a bottle of milk (which old Biff is drinking in the café when Marty arrives).
And again we reference the joke where Marty in 1955 tries to open the bottle of soda and can’t. Here, there is a flip-up straw-like device in the top.
Interesting take on the Pepsi ‘globe’ logo, and as we all know, Pepsi did change their logo significantly a few years ago.
Minor trivia: In the final cut, we don’t see Marty pay for this soda, it simply appears. He doesn’t appear to use the $50 Doc gives him. In the novelization, he tries to pay the robot waiter / Regan Headroom with cash, and the waiter seems hesitant to accept it. Later in the film, after the hoverboard chase, we see Terry ask Marty to “thumb a hundred bucks” to save the clock tower (again, har har). Which strongly suggests that your thumbprint is now tied to your bank account, and that Doc may have given Marty cash to prevent the creation of any electronic record of 1985’s Marty’s actions in town, when 2015 Marty was presumably at work (at about 4:30 PM). Some very meticulous writing there.