Ghostscript is the open source implementation of Postscript. You can read its docs online.

To compress a big PDF into something possibly smaller:

$ gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dBATCH -sOutputFile=output.pdf example.pdf

That renders basically the same PDF with all images down-res'd to 72 DPI. Other valid setting profiles are ebook, printer, and prepress.

To render a version of a PDF without any raster images in it, making it cheaper and faster to print:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dBATCH -dFILTERIMAGE -sOutputFile=output.pdf example.pdf

The FILTERIMAGE option drops raster images from the file. The FILTERVECTOR option filters vector images, and FILTERTEXT filters text.

If pdftk is not available for your OS, you can use pdf-stapler instead for cutting and concatenating PDF files. It doesn't deal with metadata as well as pdftk does, however.It's worth noting that pdftk is available as a snap package https://snapcraft.io/pdftk.

It's also worth noting that this is actually episode 2 in a series about steganography.