Acrobat Reader is the standard PDF viewer, but it takes ages to start up and every release is slower and more bloated.
A few years ago I switched to Foxit Reader. At first it was fast and nimble, but over time, it too has become slower.
My new preferred PDF viewer is PDF XChange. It’s portable, quick to start up and has reasonable memory consumption.
If you want to go for extreme performance, Sumatra is the ultimate lightweight viewer, but it’s too stripped down for my tastes.