Adobe Reader may be the most popular option for reading PDFs on your Mac, but the 77 MB app is far from being lightweight and nimble. If you are looking for an alternative that takes up less space and is easier on your computer, then you should check out PDF Reader X.
When you launch PDF Reader X for the first time, you'll immediately notice the UI, which is not as polished as some OS X apps. Its interface is functional and efficient, but that can be a benefit as a basic reader app does not require a lot of fluff. It can also be a turn off for folks who want something more than square buttons and grey backgrounds. The best part of the experience and the reason to check out the app is the tabbed interface, which allows you to open multiple documents and easily switch between them.
Beyond reading, PDF Reader X supports bookmarking and has some light annotating features like highlight, strikethrough and underline. You can also add notes, text boxes and a few other goodies. Adding notes is a little quirky as the text in the box disappears off the side of the document if you place the note at the edge of the margin. This only appears when you are editing -- the note looks fine when you are viewing the final document.
There are a few minor features like autosave which will backup your changes as you make them and a multiple-level undo that will allow you to erase your mistakes. It's all pretty standard fare for a basic PDF viewer.
PDF Reader X is available for free from the Mac App Store. It is also available on github. It's worth a download if you routinely open multiple PDFs as the tabbed interface will make switching between them a breeze.