If any of these PDF are with DRm, then you cannot convert them. That being said, given that they are PDF and if they do not have DRM, you can use pdflrf to process the PDF so they are readable on your 500. It's easier and less error prone then most PDF to LRF conversions. I've done one full conversion that turned out to have errors and I have another in the works that will have errors and won't be fully fixed till the book is read.
https://www.mobileread.com/forums/sho...ghlight=pdflrf