PDF conversion to ebook formats is very bad, here is a thread detailing some of the problems -- as well as possible solutions:
https://www.mobileread.com/forums/sho...d.php?t=118605
So you will optimally want a device that has good support for PDFs, rather than just converting them. Tablets are the best for this, really, but since that is out, I believe the Sony and Kobo readers both have better PDF readers than the Kindle does.
Here is the PDF forum, which has some useful tools for dealing with PDFs:
https://www.mobileread.com/forums/forumdisplay.php?f=184
KOReader, an alternative PDF reader for Kindle (requires jailbreak):
https://www.mobileread.com/forums/sho...d.php?t=209276