Skim

Underlining and highlighting text or making a few scribbles beside it can help you significantly if you need to study for an exam or just remember some information. While you can't do this on a computer's screen, you can use Skim to do it on digital documents, in PDF format. Even if you disregard its annotation features, the application still serves as an excellent PDF reader.

When you launch Skim, you have to open a PDF document, through the menu bar, to bring up its user interface. The loaded document is displayed in the central region of the window and its pages are listed on the left side. The application allows you to zoom and pan on a PDF's pages and it also provides a few useful options on a toolbar, like a magnifying glass or a selection tool.

The available annotation tools can be found on the aforementioned toolbar. You can use them to insert notes or anchored notes, circles or boxes, to highlight selected text, underline it or strike it out and to draw freely. All annotations you make will be listed in a column, on the right side of the interface, where you can use a search function to find a particular item. You can also right click them to reveal a few options, for copying, deleting and so on.