PDFs technically "work" on the Fire (and the Playbook, another 7" I've used recently), the issue is just that of shoving a letter/A4-sized page into half the space. You can scroll and zoom, but it's still nowhere near as comfortable as reading them at their intended size. It'd be nice to have a "smart" PDF reader that can auto-detect columns and such; but it'd never be a replacement for a proper reflowable doc format (such as ePub/mobi)
Heck, I used to read PDFs on my G1 using Repligo Reader. They have a great mode that squeezes out the vast majority of text and arranges it in a useful, readable form for the current screen.
