A lot of people who struggle with literacy simply "get by". They get relatives to help them out with tax forms or go to a tax prep service. On the job, they rely on coworkers to get them info or they do their best to try to read signs and instructions. It's rare that someone just "can't read" unless they don't speak English at all, usually it's just that they can't make out that many words and can't make sense of more complex writing. That's why pictures are so important for safety/warning signs because even someone who struggles with reading can use context clues around a big red X.