If you have the PDF open in an active window, one of control-printscreen, shift-printscreen, or alt-printscreen will copy the window to the clipboard. (I don't remember which one it is. Try them all. One will copy the entire screen to the clipboard).
Then open Windows Paint and select Copy from the Edit menu. Whatever was copied to the clipboard will appear. Crop as desired, and save as a .jpg.
Then open Windows Paint and select Copy from the Edit menu. Whatever was copied to the clipboard will appear. Crop as desired, and save as a .jpg.