Add text to a PPM image. More information: https://netpbm.sourceforge.net/doc/ppmlabel.html.
ppmlabel -x pos_x -y pos_y -text text path/to/input_file.ppm > path/to/output_file.ppm
ppmlabel -x pos_x1 -y pos_y1 -text text1 -x pos_x2 -y pos_y2 -text text2 path/to/input_file.ppm > path/to/output_file.ppm
ppmlabel -x pos_x -y pos_y -color line_color -background background_color -angle tilt -size size -text text path/to/input_file.ppm > path/to/output_file.ppm