Found a great script that does an amazing job with compressing PDF files whilst maintaining a good quality:
Get the script from here: http://www.alfredklomp.com/programming/shrinkpdf/
Just copy the script to a text file and make it executable, then run:
./shrinkpdf infile.pdf outfile.pdf
It compressed 3 PDF files from 80 MB to 13 MB!
Enjoy!
Just Some Techie Notes! 
A copy of the script:
#!/bin/sh
# http://www.alfredklomp.com/programming/shrinkpdf
# Licensed under the 3-clause BSD license:
#
# Copyright (c) 2014, Alfred Klomp
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
# 1. Redistributions of source code must retain the above copyright notice,
# this list of conditions and the following disclaimer.
# 2. Redistributions in binary form must reproduce the above copyright notice,
# this list of conditions and the following disclaimer in the documentation
# and/or other materials provided with the distribution.
# 3. Neither the name of the copyright holder nor the names of its contributors
# may be used to endorse or promote products derived from this software
# without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS”
# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.
shrink ()
{
gs \
-q -dNOPAUSE -dBATCH -dSAFER \
-sDEVICE=pdfwrite \
-dCompatibilityLevel=1.3 \
-dPDFSETTINGS=/screen \
-dEmbedAllFonts=true \
-dSubsetFonts=true \
-dAutoRotatePages=/None \
-dColorImageDownsampleType=/Bicubic \
-dColorImageResolution=$3 \
-dGrayImageDownsampleType=/Bicubic \
-dGrayImageResolution=$3 \
-dMonoImageDownsampleType=/Bicubic \
-dMonoImageResolution=$3 \
-sOutputFile=”$2″ \
“$1”
}
check_smaller ()
{
# If $1 and $2 are regular files, we can compare file sizes to
# see if we succeeded in shrinking. If not, we copy $1 over $2:
if [ ! -f “$1” -o ! -f “$2” ]; then
return 0;
fi
ISIZE=”$(echo $(wc -c “$1″) | cut -f1 -d\ )”
OSIZE=”$(echo $(wc -c “$2″) | cut -f1 -d\ )”
if [ “$ISIZE” -lt “$OSIZE” ]; then
echo “Input smaller than output, doing straight copy” >&2
cp “$1” “$2”
fi
}
usage ()
{
echo “Reduces PDF filesize by lossy recompressing with Ghostscript.”
echo “Not guaranteed to succeed, but usually works.”
echo ” Usage: $1 infile [outfile] [resolution_in_dpi]”
}
IFILE=”$1″
# Need an input file:
if [ -z “$IFILE” ]; then
usage “$0”
exit 1
fi
# Output filename defaults to “-” (stdout) unless given:
if [ ! -z “$2″ ]; then
OFILE=”$2″
else
OFILE=”-”
fi
# Output resolution defaults to 72 unless given:
if [ ! -z “$3″ ]; then
res=”$3″
else
res=”72”
fi
shrink “$IFILE” “$OFILE” “$res” || exit $?
check_smaller “$IFILE” “$OFILE”
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