pdfCropMargins – auto-detect and cut PDF margins for small-screen reading (github.com)

I find k2pdfopt cli https://www.willus.com/k2pdfopt/ to be quite comprehensive for various screen reading targets.

Briss and several forks do this as well: https://briss.sourceforge.net/. I can't remember which one I use, but it's very good.

I thought CUPS already had a way to do this.