Screen Capture

• Open the PDF file and take a screen capture of the file, by hitting "Ctrl+PrtScreen". This will copy the contents of your screen into a raster file in the system's clipboard. From there, open up your raster editor of choice (Microsoft Paint is a common one), and paste the image into it, trimming off what you don't want.
  
  

Opening a PDF in Photoshop

• From within Adobe Photoshop, open the PDF file. This will produce a dialog box with several rasterization options. You can select the color depth, including setting the document to gray scale on conversion, and select the dots per inch of the resulting raster file. You can save the file in any format that your version of Photoshop can export to.
  
  

Other Considerations

• You can replicate the Photoshop method using other high-end raster editors, such as Paintshop Pro or the GIMP on a Linux system. The screen capture method is unable (with current monitor resolutions) to create a print-scale rasterization of the PDF file.