Postscript script to add page numbers to an existing document

add_page_numbers.ps

%!PS
% originally from https://stackoverflow.com/a/57162167/13675
% Usage to concatentate multiple PDFs, number all the pages sequentially, and output a single PDF:
% gs \
%   -dBATCH -dNOPAUSE \
%   -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress \
%   -sOutputFile=/path/to/merged.pdf \
%   -f add_page_numbers.ps -f input1.pdf -f input2.pdf

% modifications by Clarence "Sparr" Risher <[email protected]>
% to add centered page numbers with prefix and suffix

% Note: Page dimensions are expressed in units of the default user space (72nds of an inch).
% inspired by https://www.ghostscript.com/pipermail/gs-devel/2005-May/006956.html

globaldict /MyPageCount 1 put % initialize page counter

% String concatenation function from ghostscript Resource/Init/gs_init.ps
/concatstrings % (str1) (str2) concatstrings (str1str2)
{ exch dup length 2 index length add string % str2 str1 new
dup dup 4 2 roll copy % str2 new new new1
length 4 -1 roll putinterval
} bind def

% executed at the end of each page. Before calling the procedure, the interpreter
% pushes two integers on the operand stack:
% 1. a count of previous showpage executions for this device
% 2. a reason code indicating the circumstances under which this call is being made:
%    0: During showpage or (LanguageLevel 3) copypage
%    1: During copypage (LanguageLevel 2 only)
%    2: At device deactivation
% The procedure must return a boolean value specifying whether to transmit the page image to the
% physical output device.
<< /EndPage {
  exch pop % remove showpage counter (unused)
  0 eq dup { % only run and return true for showpage
    MyPageCount 0 lt % starting page of "later pages" below
    {
      /Courier 8 selectfont % select font and size for early pages
    }
    {
      /Courier 12 selectfont % select font and size for later pages
    }
    ifelse
    MyPageCount =string cvs % get page counter as string
    (PREFIX) exch concatstrings % add prefix to page number string
    (SUFFIX) concatstrings % add suffix to page number string
    dup % need it twice (width determination and actual show)
    stringwidth pop % get width of page counter string ...
    2 div % halve the width of the string
    currentpagedevice /PageSize get 0 get % get width from PageSize on stack
    2 div % halve the width of the page
    exch sub % pagewidth/2 - stringwidth/2
    18 moveto % move to calculated x and y=18 (0/0 is the bottom left corner) (18 units is 1/4")
    show % finally show the page counter
    globaldict /MyPageCount MyPageCount 1 add put % increment page counter
  } if
} bind >> setpagedevice