PDFs are just a terrible eBook format. They bake in a fixed page size, usually 8.5x11 inches.

The Kindle's screen being physically smaller than that, your display options are limited to scaling an image of the 8.5x11 page down to fit into 4.8x3.6 screen, at which point fonts are generally too small to read, or attempting to reflow the PDF's contents on-the-fly, which is noticeably slow and typically only gives acceptable results if the PDF contained an absolute minimum of formatting.