Quality level settings are /screen, the lowest resolution and lowest file size, but fine for viewing on a screen; /ebook, a mid-point in resolution and file size; /printer and /prepress, high-quality settings used for printing PDFs.

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/printer -sOutputFile=output.pdf input.pdf