[JavaBrewer]

On a PC I always keep a blank bitmap image on my desktop (Right click on desktop and select New->Bitmap Image)

Open your PDF file
With your PDF application open and selected do a ALT-PrintScreen
Right click your blank bitmap image and select Edit
Paste from clipboard to image.
Save as whatever you wish.

On a Mac

Press Command (⌘)-Shift-4. You'll see that your cursor changes to a camera pointer.
Press the Space bar.
Move the camera pointer over the window to highlight it.
Click your mouse or trackpad. Or to cancel, press Escape (esc) before you click.
Find the screenshot as a .png file on your desktop.