There are very few rigorous measurements and a lot of people just saying whatever they feel like :)
With regards to acid and GERD which is when doctors normally talk about coffee acidity, what they usually say makes no sense at all and they should know better.
The idea that a very weak acid like coffee would increase the acidity of your stomach which contains a very very strong acid (1.5 to 3.5) is ridiculous.
I think what they should just say is that hot coffee can relax the muscles keeping your stomach contents in.
