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.
if all you have is a pdf, it very likely will not convert nicely to epub without some work on your end. if you're just trying to view a textual document, like a novel or something, the big advantage of an epub is that it'll reflow to fit your screen. you can change the font size or style, margins, line spacing, whatever, and its like the book was just printed that way, and you can flip pages naturally. if your pdf has a small font, you've got to scroll and zoom and it is a big pain in the butt. pdfs are designed for printing first and foremost, while epubs are meant to be read digitally.