Perhaps the best way is to not convert at all.
- Conversion always risks some undesirable alteration
- PDF is a beautiful format for its intended use (onscreen emulation of a printed document), but a horrid interchange format. Don't use is as a middle-man.
My suggestion is to open the Kindle/Mobi/AZWn book in calibre's ebook-viewer. You should be able to select the text there, copy, and then paste into you desired target app.