#!/bin/bash

mkdir -p bak
mv $1 bak

echo "%!PS-Adobe-2.0
%%BoundingBox 0 0 594 765
/Times-Roman findfont 96 scalefont setfont
170 200 moveto 40 rotate 1 1 scale
0.85 setgray ($2) show
showpage" | ps2pdf - watermark.pdf
pdftk bak/$1 background watermark.pdf output $1
rm watermark.pdf