Have some library books you want to put on your kindle? Disappointed because most libraries have encrypted PDFs, which cannot be read on the kindle? You may not need to spend $100 for a New York Public Library card. Depending on what you like to read, your free public library may have all the books you want to read, except they are in PDF.
Well, look here:
https://www.mobileread.com/forums/showthread.php?t=40238
BTW, I have found after decrypting, Mobipocket Creator was the easiest to reformat for kindle.