Linux – Easiest method to create pdf from template using command line (without pdflatex)

Well, I've come up with a solution using TikZ within a crafted LaTex document. The result is not exactly the same, but I think it is even nicer:

This required having a tex document with placeholders that will be replaced by the arguments to a sh script.

% file: add_legend.tex

\documentclass{standalone}
\usepackage{graphicx}

\usepackage{tikz}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% LaTeX Overlay Generator - Annotated Figures v0.0.1
% Created with (omitted http) ff.cx/latex-overlay-generator/
% If this generator saves you time, consider donating 5,- EUR! :-)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\annotatedFigureBoxCustom{bottom-left}{top-right}{label}{label-position}{box-color}{label-color}{border-color}{text-color}
\newcommand*\annotatedFigureBoxCustom[8]{\draw[#5,thick,rounded corners] (#1) rectangle (#2);\node at (#4) [fill=#6,thick,shape=circle,draw=#7,inner sep=4pt,font=\huge\sffamily,text=#8] {\textbf{#3}};}
%\annotatedFigureBox{bottom-left}{top-right}{label}{label-position}
\newcommand*\annotatedFigureBox[4]{\annotatedFigureBoxCustom{#1}{#2}{#3}{#4}{white}{white}{black}{black}}
\newcommand*\annotatedFigureText[4]{\node[draw=none, anchor=south west, text=#2, inner sep=0, text width=#3\linewidth,font=\sffamily] at (#1){#4};}
\newenvironment {annotatedFigure}[1]{\centering\begin{tikzpicture}
\node[anchor=south west,inner sep=0] (image) at (-0.75,-0.75) { #1};\begin{scope}[x={(image.south east)},y={(image.north west)}]}{\end{scope}\end{tikzpicture}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}  

  \begin{annotatedFigure}           
        {\includegraphics[width=1.0\linewidth]{_image_}}    
        \annotatedFigureBox{0,0}{0.000,0.0}{_letter_}{0,0}%bl
    \end{annotatedFigure}  

\end{document}

And the sh script:

#!/bin/sh
# Call this script with at least 2 parameters, for example
# sh scriptname <image_file> <letter_of_legend> 

cat add_legend.tex | sed "s/_image_/$1/g" | sed "s/_letter_/$2/g" | pdflatex

#rename output to match <letter_of_legend>_<image_file> format
mv texput.pdf $2_$1 

#clean up
rm texput.*

exit 0

Finnaly, by calling:

$> ./legend.sh online_gauss.pdf A

the output drawn in "A_online_gauss.pdf"!

convert, the ImageMagick utility used in Ketan's answer, also allows you to write something like

convert xc:none -page Letter a.pdf

or

convert xc:none -page A4 a.pdf

or (for horizontal A4 paper)

convert xc:none -page 842x595 a.pdf

etc., without creating an empty text file. @chbrown noticed that this creates a smaller pdf file.

"xc:" means "X Constant Image" but could really be thought of as "x canvas". It's a way to specify a single block of a color, in this case none. More info at http://imagemagick.org/Usage/canvas/#solid which is the "de facto" manual for ImageMagick. [supplemented with information from pipe] (Things like pdf:a can be used to explicitly declare the format of a file. label:'some text', gradient:, rose: and logo: seem to be other examples of special file formats.)

Anko suggested posting this modification as a separate answer, so I am doing it.