I only put PDFs I created myself on my Kindle, so I can create them with
a page size that matches the screen.
The PDF viewer on Kindle can't - as far as I'm aware - zoom, but only display the PDF fit-to-width, and most PDFs are created with a larger page size. It makes the PDF viewer very limited - but I'm still glad it's there. With some documents, such as recipes or travel itineries, it's nice to be sure that a certain set of information is displayed on one page.
Edit: almost wrote 'time travel itineries'

- when will technology catch up?