When speaking of a coffee's flavor the word "acidity" is often used. However, this is misleading and only refers to the flavor. Chemically, coffee's pH is generally about neutral (similar to plain water), neither acidic or basic, depending on the bean and the roasting. Most fruit juices have a higher acid content.
Coffee can sometimes stimulate our stomach to produce more acid. You can avoid the irritable stomach feeling occasionally associated with drinking coffee on an empty stomach. It is often as simple as picking a darker roast. The darker roasts or even drinking espresso can resolve the issue. Not only are the darker coffees lower in over-all acidity, but they tend to not stimulate acid production as much. Of course, eating something will also alleviate the sensations.