We use pdftk here, pdftk stands for PDF toolkits, is a cross-platform tool for manipulating PDF files.

How to install pdftk?

$ sudo pkg install pdftk # on FreeBSD
$ sudo apt-get install pdftk # on Debian / Ubuntu based GNU/Linux

How to compress PDF via pdftk?

$ pdftk input.pdf output output.pdf compress

You can also decompress by replacing ‘compress’ keyword with ‘uncompress’

$ pdftk input.pdf output output.pdf uncompress

That’s all. Notes that the compression usually works with images only, may not work on the file only contains text and simple shapes.