How to add transparent text watermark to PDF / PS file

OK, I see. Thank you very much for your reply.

In fact, I found a PS script to generate transparent effect, but I can't combine it with other PDF.

Examples are as follows:

%!PS
%   Postscript that uses Ghostscript extensions to do simple transparency

<< /CompatibilityLevel 1.4 >> setpagedevice % in case we are going to pdfwrite

% work around rectfill mapping directly to device fill_rectangle
/rectfill {
  gsave 4 2 roll moveto 1 index 0 rlineto
  0 exch rlineto neg 0 rlineto closepath fill grestore
} bind def 

0 .pushpdf14devicefilter % depth .pushpdf14devicefilter -
     << >> clippath pathbbox newpath .begintransparencygroup

     .5 .setopacityalpha 1 .setshapealpha

     .5 .9 .2 setrgbcolor
     0 0 300 700 rectfill
    
     .2 .5 .9 setrgbcolor
     200 100 400 400 rectfill

     .9 .3 .1 setrgbcolor

     100 200 500 500 rectfill
     .endtransparencygroup
.poppdf14devicefilter

showpage
quit