I'll open Mail and create a new email message. Into it I'll drag a PDF file that I'd like to mark up. I'll click on the triangle icon that appears at the top of the file and choose Markup.
A separate pane appears where I can choose my tools.
If this toolbar looks a little familiar, it should. Many of the tools in it were pulled from the Preview app, which also allows you to annotate images and PDFs.
The Sketch tool lets you scribble on a document, but it's also smart about it. For example, if I draw a rough word bubble, Markup will offer to turn it into a real one--or make it a perfect circle if I like, or just leave it as it was originally drawn. Once I've drawn something, I can change its line width, its color, and choose to fill it with color if I like.
The Shape tool next door lets me insert lines and shapes. I can also add a highlighter rectangle that makes everything outside of it dimmer.
And then there's the Magnifier. Just select it and drag it to where you'd like. You can make it smaller or larger as well as adjust the degree of magnification.
If you like working with words, choose the text tool and start typing. Click on the Text option to change the font, size, color, formatting, and justification of your text.
And finally, you can sign your work. Just click on the Signature tool and hold your written signature up in front of your Mac's camera. Just as with the Preview app, Markup will recognize your signature. Add it and you can then append it to the file.