When you're messing around with a PDF in Preview, you may need to select elements like text or images. There's an easy but fairly non-obvious way that you can control what's being selected. The secret lies in two toolbar icons—the Text Selection and Rectangular Selection buttons. To reveal these useful features, the first thing you'll do is click on the "pen in a box" icon in your toolbar, which is the Show Edit Toolbar button.
…to reveal tons more options.
I've highlighted the two choices we're gonna cover—the Text Selection button is the A in the square, and the Rectangular Selection button is the dotted-line square. If you'd prefer, you can access these options under the Tools menu, too.
Now that we've found our goodies, let's go over Text Selection first. So when you click the Text Selection button to enable it (it'll turn blue), you can then click and drag within your PDF to select any text.
Obviously, this is nifty if you need to copy and paste text from a PDF into a different document. Another quick tip on this is that if you hold down the Option key before you start dragging, you can draw a box around the text you want to grab instead.
See the little box around my text?
That'll make it much easier to grab text that's in funky, difficult-to-select columns or what have you.
So that's what the Text Selection feature has to offer, but I find Rectangular Selection even more useful. Once you toggle that button on in your toolbar, you can enclose pretty much anything—a single image or multiple graphics, text with images, and so on—in the rectangle you click and drag, and it'll be captured as a selection.
As you can see, I've selected a header with some text and an image. If I then copy what I've got selected, I have a ton of choices. I can paste it into a word-processing program as a graphic, insert it into the body of an email, or (my favorite trick) press Command-N (File> New from Clipboard) in Preview to start a whole new document with just my selected area.
From there, I can save it out in all sorts of formats and manipulate it any way I want to. So it's simple to grab any of the elements of a PDF for use elsewhere! Sweet. If I could take a program out to dinner, Preview'd be high up on my list, let me tell you. Is that weird?