In the open source world, the best choice for PDF command line foo (and PDF foo in general) is almost always ghostscript. This is a quick way to extract a single page from a PDF file and save it as PNG file with a given (resolution in dpi):

#!/bin/bash
INFILE="$1"
OUTFILE="$2"
PAGE="$3"
RES="$4"
 
gs -dBATCH -dNOPAUSE -sDEVICE=png16m \
    -r$RES \
    -dFirstPage=$PAGE \
    -dLastPage=$PAGE \
    "-sOutputFile=$OUTFILE" \
    "$INFILE"

Example usage:

./pdf_page_to_png.sh input.pdf output_p3.png 3 200