Typically there is a basic browser, it would not be the preferred way to read things.
They can do pdf but it’s not ideal. PDF is a specific page size, it is better if the text is rendered by the device itself. EPUB is a pretty common format for books (just not on kindle), but there are other formats including plain text. A site like Standard Ebooks has really nice, free content. Lots of libraries have ebooks too.
E-readers are much better for reading. The battery lasts forever because it is not keeping a display “on” most of the time. On a good one the text is nearly indistinguishable from text on a printed page - but with resizable and changeable fonts. The writing is actually on a physical medium that is repositioned with an electric charge, it’s probably not like any other screen you’re used to.