Use gs or ghostscript as below, specify the output file and also the input file.
In the example below the output file is "outputfile-resized.pdf" and the input file is "original-pdf.pdf". Change the input file to the name of your pdf
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH -sOutputFile=outputfile-resized.pdf
original-pdf.pdf
For example the above method took a pdf from 12MB to about 500KB with no noticeable quality loss.
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