When the dimension of your PDF page exceeds the dimension of the PDF Snipping Tool window, you are suggested to use mouse panning to better view your PDF page.
What is mouse panning?
1) Click Zoom In 
button twice so as to make your PDF page large enough for panning.
2) Move your mouse cursor to the center of your PDF document, and press down the left button on your mouse. Move your mouse to the left or to the right, up or down, while keeping the left button down.
3) Release the left button on your mouse.
If mouse panning does not work right for you, check the following,
1) Is your PDF page magnified as large as needed?
2) When you move your mouse cursor holding the left button on your mouse down, is the move far enough?