Screens have a lower resolution than a printed page, so larger font sizes and simpler font designs are needed for easy reading. The whole of a page designed for printing will not fit on today's computer screens. The reader has to clumsily scroll back and forth across a line to read the screen. The disabled have an added problem with increasing the font size further, or using Braille devices. In contrast web browsers designed for HTML don't have a page size built in.
Text wraps to fit the screen size.
The font style and size can be selected by the reader. Options allow the disabled requirements to be met. Web pages use hypertext links to break up a large document into components. PDF documents can use the book metaphor of chapters, but all chapters are stored within the one large document. This can make reading one chapter of a large document a very slow process.