Removing watermarks from a PDF document under OS X

#!/bin/sh

# USAGE: removewatermark file.pdf
# outputs to $OUTDIR/file.pdf

# How to use this script to remove watermarks from PDF documents on OS X (MacOS):
#
# 1. port install qpdf
#
# 2. normally, you would also run "port install pdftk" but that doesn't work
# on os X 10.11+ as of 2016-11, so instead download the installer from pdflabs
# referenced in http://stackoverflow.com/a/33248310.
#
# 3. Write a program called "stripwm" that removes all watermarks from the
# uncompressed pdf read from standard input and writes the result to standard
# output. Save it in the current directory, or change the STRIPWM variable
# to point to its location. See below for an example.
#
# References:
#
# http://superuser.com/questions/455462/how-to-remove-a-watermark-from-a-pdf-file

OUTDIR=fixed
STRIPWM=./stripwm
DECRYPTED=$(mktemp /tmp/fixit.XXXXXX)
UNCOMPRESSED=$(mktemp /tmp/fixit.XXXXXX)
DONE=$(mktemp /tmp/fixit.XXXXXX)
qpdf --decrypt "$1" "${DECRYPTED}"
pdftk "${DECRYPTED}" output "${UNCOMPRESSED}" uncompress
"${STRIPWM}" <"${UNCOMPRESSED}" >"${DONE}"
pdftk "${DONE}" output "${OUTDIR}/$1" compress

# Here is an example to remove watermarks from MAA books:

<<STRIPWM
#!/usr/bin/perl

$_ = join("", <>);

s~
(\n\d+\s+\d+\s+obj\s+
<<\s+
/Length\s+\d+\s+
>>\s+
stream\s+)
Q\s+
q\s+
BT\s+
0\s+0\s+0\s+rg\s+
[^\n]*Tf\s+
[^\n]+Tm\s+
[^\n]*Your\s+Name[^\n]*Tj\s+
[^\n]*Tm\s+
ET\s+
Q\s+
(endstream\s+
endobj[^\S\n]*\n)~$1$2~sgx;

print;
STRIPWM