Have Ghostscript installed, and then

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=compressed_output.pdf input.pdf

The possible -dPDFSETTINGS are:

• /screen selects low-resolution output similar to the Acrobat Distiller "Screen Optimized" setting.
• /ebook selects medium-resolution output similar to the Acrobat Distiller "eBook" setting.
• /printer selects output similar to the Acrobat Distiller "Print Optimized" setting.
• /prepress selects output similar to Acrobat Distiller "Prepress Optimized" setting.
• /default selects output intended to be useful across a wide variety of uses, possibly at the expense of a larger output file.

While there are online services that can compress such documents, these are of course not suitable for privacy-sensitive documents and may in addition insert malware into the created documents.