How to export Kindle notes/highlights to a PDF?

That is easy, you have several options:

1. Visit https://read.amazon.com/notebook, you can browse all notes and highlights of all your Kindle books there, and export selected ones to PDF.
2. You can use the onlien tool clipping.io, here is the guide: https://www.ereader-palace.com/how-to-transfer-kindle-highlights-to-computer/
3. You can use a desktop software called Knotes: http://knotesapp.com/, and there is an alternative called Klib: http://klib.me/