Today I had to compress a very hard to compress #PDF, so I have some tips I guess if you find yourself with a PDF filled with huge images that you have to shrink by half (and you don't mind losing text selection).
First thing is to open it on #Scribus, and you can try saving it as PDF again with some compression options... might be enough for you... (and you'll keep the text selection)
But for me I had to export each page as a 200dpi PNG and then I wrapped it back as a PDF using
#img2pdf (https://gitlab.mister-muffin.de/josch/img2pdf)
pip install worked for me... the experimental GUI didn't work so I navigated the terminal to my images folder and used:
$ img2pdf --output out.pdf --pagesize A4 *.png