With Preview, you can open your PDF files and do some modifications to the files. Modifications can be done only when file isn't locked.
There are several useful tools in Preview that you can use. One of those is the Mark Up Tools.
As the name suggest, this tool is useful for you to emphasize some parts of your PDF files. There are three mark up tools available: Highlight Control Command H, Strike Through Control Command S and Underline Control Command U. Don't worry about the shortcuts since they're easy to remember: take out the first letter and add Control Command in front.
If you still forgot what are they, you can always refer back to Preview menu Tools ▸ Mark Up. The tools can be seen there.
It's really simple to use these tools. You only need to draw a selection, hit the keystrokes, and the part under the selection will be marked.
For the sake of completeness (and for the sake of avoiding short article), I will introduce you a killer trick to draw a rectangular selection in Preview.
Hold down Option key, move pointer a bit and the pointer should be in Crosshair mode now. After that, you can draw a region (like the one in screen capture mode) and all texts under the region will be selected. This trick is amazingly useful when you're marking up tabular kind of data.
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