Convert pdf to monochrome

From StackExchange

gs \
 -sOutputFile="BW-$1" \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -dProcessColorModel=/DeviceGray \
 -dCompatibilityLevel=1.4 \
 -dNOPAUSE \
 -dBATCH "$1"

Writes sample.pdf to BW-sample.pdf