source : https://www.smashingmagazine.com/2015/06/efficient-image-resizing-with-imagemagick/

#!/bin/bash
if [ $# -lt 3 ];then
  echo $"missing arguments"
  exit 1;
fi

# arg 1 : source file
if [ ! -f $1 ]; then
	echo $"file $1 not found"
	exit 1;
fi

# arg 2 : width
re='^[0-9]+$'
if ! [[ $2 =~ $re ]] ; then
   echo "error: Not a number" >&2; exit 1
fi

# arg 3 : output folder
if [ ! -d $3 ]; then
	echo $"folder $3 not found"
	exit 1;
fi

mogrify -path $3 -filter Triangle -define filter:support=2 -thumbnail $2 -unsharp 0.25x0.08+8.3+0.045 -dither None -posterize 136 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB $1

use :

for filename in *.jpg; do optimizer.sh $filename 1024 outfolder;done

note : https://stackoverflow.com/questions/52998331/imagemagick-security-policy-pdf-blocking-conversion

edit /etc/ImageMagick-6/policy.xml

<policy domain="coder" rights="read | write" pattern="PDF" />

and then

convert *.jpg mypdffile.pdf

#!/bin/bash

# Script to convert PDF file to JPG images
#
# Dependencies:
# * pdftk
# * imagemagick

PDF=$1

echo "Processing $PDF"
DIR=`basename "$1" .pdf`

mkdir "$DIR"

echo '  Splitting PDF file to pages...'
pdftk "$PDF" burst output "$DIR"/%04d.pdf
pdftk "$PDF" dump_data output "$DIR"/metadata.txt

echo '  Converting pages to JPEG files...'
for i in "$DIR"/*.pdf; do
  convert -colorspace RGB -interlace none -density 300x300 -quality 100 "$i" "$DIR"/`basename "$i" .pdf`.jpg
done

echo 'All done'