No, it doesn't bog down your Kindle. ((Thou shalt not use my name as a verb

)) Amazon converts the pdf to a text type document. The Kindle cannot read pdf's. I've tried. You mail the pdf to your special
bogusar@kindle.com or whatever, Amazon converts it for you and they send it to your Kindle. It's brilliant. By the way, you have to "allow" email addresses to send stuff to your Kindle. It won't accept emails from just anybody. That way you don't get spammed.
Besides, I have many converted pdf books on my Kindle and I'm not experiencing any "bogging down". If you need more space then there's an SD card reader on it. I don't have an SD card in mine and I have plenty of space. Any book you've ever bought from Amazon is saved by them on your account so if you delete some, or lose your Kindle, you can always re-download them. And if you have converted files you want to save you can always use a USB cord to hook it up to your computer and save them on your hard drive. You can hold tons of books on just the memory it comes with anyway.
I've noticed that too, that only some of an author's books are available. I don't know why.
--bog