Windows only: Sumatra PDF has always focused on being the faster, lighter and less system-burdening alternative to Adobe and other PDF solutions, and its latest 1.3 update continues down that path. Images are rendered faster, in particular, and less memory is used.
You can read up on all the core changes to the app at the changelog linked below, but note that, along with speed and performance fixes, Sumatra also works more like Adobe in a few ways. Copying and manipulating text is easier, and there are a few options for scrolling through documents, going full-screen, and selecting multiple images.
Sumatra PDF is a free download for Windows systems only.
News about Sumatra PDF reader [Sumatra PDF]