Here's the code -- pretty simple:

    #!/bin/sh
    
    # by Jason A. Donenfeld
    # www.zx2c4.com
    
    if [ $# -ne 2 ]; then
        echo "Usage: $0 url output-pdf-file"
        exit 1
    fi
    
    set -e
    documenturl="$(echo -n "$1" | xxd -plain | tr -d '\n' | sed 's/\(..\)/%\1/g')"
    viewerurl="http://docs.google.com/viewer?url=$documenturl"
    pdfurl="$(printf "$(curl -s "$viewerurl" | sed -n "s/.*gpUrl:'\\([^']*\\)'.*/\\1/p" | sed 's/%/%%/g')")"
    cookiejar="$(mktemp)"
    curl -s -L -c "$cookiejar" -o "$2" "$pdfurl"
    rm -f "$cookiejar"