How to convert a color pdf to black-white?

I'd like to transform a pdf with some coloured text and images in another pdf with only black&white, in order to reduce its dimensions. Moreover, I would like to keep the text as text, without transforming the pages elements in pictures. I tried the following command: convert -density 150 -threshold 50% input.pdf output.pdf found in another question, [a link](https://unix.stackexchange.com/questions/84709/how-can-i-convert-a-pdf-file-from-gray-scale-to-black-white) , but it does what I don't want: the text in the output is transformed in a poor image and is no longer selectable. I tried with Ghostscript: gs -sOutputFile=output.pdf \ -q -dNOPAUSE -dBATCH -dSAFER \ -sDEVICE=pdfwrite \ -dCompatibilityLevel=1.3 \ -dPDFSETTINGS=/screen \ -dEmbedAllFonts=true \ -dSubsetFonts=true \ -sColorConversionStrategy=/Mono \ -sColorConversionStrategyForImages=/Mono \ -sProcessColorModel=/DeviceGray \ $1 but it gives me the following error message: ./script.sh: 19: ./script.sh: output.pdf: not found Is there any other way to create the file?