[tempay]

No you don't, HTML has massive variation between devices and what software is used to display it. Try setting your screen resolution to 640x480 and opening a webpage or, even worse, modifying DPI. PDFs on the other hand specify exactly where to place each glyph (admittedly there is still variation between software but it's much more consistent).

[dahauns]

You sure do - the variation between devices is the manifestation of the ability to control layout depending on the recipient's display's resolution and size.

If anything, a PDF designed for A4/letter is going to be cumbersome to read on a (probably rather small) 640x480 display.

The sad state of PDF rendering on (most) e-book readers should be evidence enough.