En étudiant le watermarking,
nous avons constaté que très peu de travaux ciblaient les documents PDF
qui représentent cependant une part non anecdotique des données
échangées en ligne.


Several methods of  Steganography and Digital Watermarking  in PDF and
Text documents have been proposed. In~\cite{PD2008}, a steganographic approach
is   presented   by   hiding    information   using   inter-word   and
inter-paragraph  spacing in  a  text. The  main  disadvantage of  this
method is that the hidden message  can be destroyed by simply deleting
some  spaces between  the  words  in the  stego  text.  In~\cite{PD2008},
two
different  algorithms   are  proposed  which  are   considered  as  an
alternative  for the  original  TJ operator  method.  The TJ  operator
displays  the  text  string  in  a  PDF  document,  allows  individual
character positioning  and uses character and  word spacing parameters
from  the  text  state.  The alternative  method  has  less  embedding
capacity than the  original method. In~\cite{LLGC13}  an encryption technique
is  proposed by  combining  the information  hiding  technique in  PDF
documents and  the quadratic  residue as  basis and  then apply  it to
copyright protection and  digital learning. The main  drawback of this
method is  that the hidden  message can be  easly removed. In~\cite{DBLP:journals/sigpro/LeeT10}, an
embedding method in  source programs using invisible  $ASCII$ codes is
proposed. This method is very easy  to detect by simply extracting the
modified  text  from  the  document,  converting  it  to  hexadecimal,
extracting all  the inserted  invisible $ASCII$ characters,  and then,
decoding the embedded message.  In~\cite{WT08}, a data hiding in PDF files and
applications by  imperceivable modifications of PDF  object parameters
is proposed. This  method serves to hide data  by slight modifications
of the values  of various PDF object parameters such  as media box and
text   matrices.  The   method  is   considered  to   have  sufficient
transparency  while  its  main  drawback is  its  very  low  embedding
capacity.

Substitutive   Quantization  Index   Modulation  (QIM)   methods  were
introduced  by Chen  and Wornell~\cite{CW01}. The  Spread Transform  Dither
Modulation (STDM) is an implementation of  this scheme and it has been
considered  robust  under  different watermarking
attacks~\cite{DM10,WLSYNW13,CW99}.

In this  paper, the goal  is to  present a blind  digital watermarking
scheme for PDF documents based on  a variant of the Quantization Index
Modulation   method   called   Spread  Transform   Dither   Modulation
(STDM). The main difficulty in PDF  documents is to find a significant
watermarking  space in  order  to  embed the  secret  message under  a
sufficient Transparency-Robustness tradeoff. Our contribution consists
in using  the $x$-coordinates of a  group of characters to  embed each
bit  of  the  secret  message  while  choosing  the  appropriate  mean
distortion value which gives  the strong tradeoff between transparency
and robustness.