Screen readers are a type of assistive technology that is able to read aloud text that is on a screen. They are designed for people who experience visual or cognitive disabilities or who are otherwise unable to visually read digital material.
While screen readers can read simple, plain text with ease, it is more difficult to read information that is presented in columns, tables, or charts, since the screen reader can only read right to left, up and down, straight across a page, unless it is instructed to do otherwise.
However, digital tags can be added to the background HTML to tell a screen reader how and where to read and in what order. The same is true of images and PDFs, which are inaccessible to screen readers because they only read text. PDF files can be converted to accessible formats and alt text can be added to images to make them accessible.
This short video explains a bit about assistive technology users, shows how screen readers work and demonstrates the use of a screen reader with both an inaccessible and an accessible PDF document.