Test out other, free PDFs first...
My wife has the 7-inch HD kindle fire which she uses for 4e gaming and reading (in addition to many other things). Ultimately whether or not you find PDFs readable on the screen is up to your own personal preference. Screen real estate is limited compared to a monitor so you'll only be able to read snippets at a time, but the search feature is your friend and will allow you to jump anywhere.
In general comparison between PDFs and Rulebooks.
PDFs are much faster and easier to use (and of course eminently more portable) when looking up specific information. If you have a rules question or need to reference a specific feat or item, the PDF will always be quicker than the book. Likewise you can easily make an archive copy of the PDF to store on your personal google drive or skydrive to access from anywhere.
Books on the other hand look and feel nice. They are nice to display on shelves and can be more enjoyable to use if you are just browsing or using them for a character creation only "zero session" as players can look at more options at once.
If you're unsure which you'll prefer, there's plenty of free Paizo content that you can test drive, like this Rise Of The Runelords Player's Guide
