If you from some reason need to convert your PDF document into multiple images such as PNG or JPG you can use Imagemagick suite to do this job. First create a directory and place there you PDF document you wish to convert.
Copy your PDF document into this directory and run a following command.
The command above will create multiple transparent png images. The number of images depends on number of pages in you original PDF document. The option resize means that all output images will have a width of 500px. Feel free to adjust this size or even omit it if necessary.
In case that you do not wish to have a transparent background images change to above command to:
or convert all your png images and set white background for all new created image using simple bash for loop:
All images with a prefix bg_ will now have white background.