Try k2pdfopt! I use it all of the time with scientific papers, with lots of formulas, and it works quite well. It practically converts the pdf to images and slices them up, outputting another pdf, but the size increase is not too significant (still usable file sizes with multiple-hundred page long books).
Thanks! This isn't actually useful to me since I read almost nothing really hardcore on my phone but it's good to know about.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.