Rainwater is usually fine to drink, but be careful that it hasn't been in contact with anything poisonous on its way out the sky - particularly dripping off plants etc might pick up toxins.
As to acidity and chemical content - the UK average pH of rainwater is around 5 - which is more acidic than 'pure' water but much higher than other drinks (cola is usually around 2.5 to 3!)
As far as chemicals go, I believe that you can avoid a lot of these if you don't drink rainwater that has only just started falling, as this will be highest in particulates etc that its hitting and taking out the air.
However, I've just found this link
http://www.kitchendoctor.com/reprints/water.html which seems to imply that there's too many pesticides and herbicides in rainwater in Europe to make it drinkable...