Ghostscript

gs -dNOPAUSE -dQUIET -dBATCH -sDEVICE=pdfwrite -sOutputFile=output.pdf input1.pdf input2.pdf

gs -dNOPAUSE -dQUIET -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -sOutputFile=output.pdf input.pdf
gs -dNOPAUSE -dQUIET -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dDownsampleColorImages=true -dColorImageResolution=72 -sOutputFile=output.pdf input.pdf

Ghostscript
https://www.ghostscript.com/

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