On my Mac I would just use Apple’s Preview which open’s PDF files, and on the Kindle their standard app.
On the iOS devices I am preferring the Kindle app to Apple’s ‘Books’ app - partly because it’s been a nightmare trying to sync books on to all devices!
On the Kindle app you just need to email yourself a copy of your book and it will appear on all connected devices, and it even syncs your notes and last read page - so you can continue reading on any device. It’s also more customisable and just a bit more polished imo.
What about you? What do you use?