https://en.wikipedia.org/wiki/PDF

# unencrypt .pdf file
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET -dBATCH -sOutputFile=./my_unencrypted_output.pdf ./my.pdf

# Extract pages 1 to 13 into separate files - or https://www.ilovepdf.com/split_pdf
pdfseparate -f 1 -l 13 ./my.pdf ./tmp_page_%d.pdf

# Unite all *.pdf files into one - or https://www.ilovepdf.com/merge_pdf
pdfunite $(ls -la ./tmp_page_*.pdf | awk '{print $9}' | sort -V) ./my_output.pdf && rm -f ./tmp_page_*.pdf

# Compress resulting PDF - this produces better results, by far: https://www.ilovepdf.com/compress_pdf
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH -sOutputFile=my_compressed_output.pdf my_input.pdf

Or, online services that can do this:

  1. ilovepdf.com - split, merge, convert, etc.
    1. compress_pdf - compress (even embedded images are reduced in quality)
  2. pdfcompressor.com (can not compress as good as ilovepdf.com)