The issue with reading outdoors is the glare from the sun. polarized reading sunglasses are the best at reducing glare and letting you read even without having to find shade. Even when you are directly under the sun, you can keep reading with you reading sunglasses.
Make Sure You Are Protected
Your eyes must be protected all the time even when you are not reading outdoors. When you are outdoors, the sun's rays is danger to your hypersensitive eyes. When looking for reading sunglasses, ensure you ask the manufacturers or the sellers if they have high defense against UVB as well as UVA rays.