I read a LOT of PDFs (technical documents) on my Kindle Gen2. Never tried to convert anything. Most of the books I read are out-of-copyright classical works that I get from Google or my local library.
Some proprietary formats have to be converted, others read as they are. My wife has a Nook(EPUB), I have a Kindle(MOBI). Neither reader can read the format of the other. Since we both read a lot of books, I just convert them either way, no fuss, no muss.
You can read PDFs natively on the Kindle, conversion can make them more friendly to the Kindle.