monochrome.sh

 #!/bin/bash
COUNTER=1
PAGES=1168
while [  $COUNTER -lt $PAGES ]; do
    TO=$((COUNTER+100))
    if [ $TO -gt $PAGES ]
    then
	let TO=$PAGES
    fi
    
    /snap/bin/pdftk Breviarium.pdf cat $COUNTER-$TO output breviarium/Breviarium$COUNTER-$TO.in.pdf
    convert -density 600 -threshold 61% breviarium/Breviarium$COUNTER-$TO.in.pdf breviarium/Breviarium$COUNTER-$TO.out.pdf
    echo $TO/$PAGES
    
    let COUNTER=$((TO+1))
done