zlacker

ASCII PPM supports comments, so it is possible that EXIF or other identifying information would get written into the comments by some tool.

I have only ever observed PPM comments right at the start of the file, so you could open it in a text editor and remove the comments from the start. Maybe check the very end of the file as well.

Binary PPM does not support comments, so that would be a better solution. PPM documentation here, you want possibly P3 or more likely P6 https://en.wikipedia.org/wiki/Netpbm#File_formats