One liner with ghostscript GS to make your pdf much smaller. Options for PDFSETTINGS are (in rank of output quality):

1. /prepress
2. /ebook
3. /screen

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH -sOutputFile=compressed_PDF_file.pdf input_PDF_file.pdf

Resources

https://itsfoss.com/compress-pdf-linux/