Re : simple IPimage to PDF doesn't work

You changed according to my first remark, not to my second one. All drawings on the PDF surface must be prefixed by the PDF PGraphics name.

1. PImage img;
2. import processing.pdf.*;
3. PGraphicsPDF my_pdf;
4. void setup(){
5.   size(500, 375);
6.   img = loadImage("15.jpg");
7.   my_pdf = (PGraphicsPDF)createGraphics(500, 375, PDF, "img.pdf");
8.   my_pdf.beginDraw();
9.   my_pdf.image(img, 0, 0);
10.   my_pdf.endDraw(); // stop and save pdf
11.   my_pdf.dispose();
12.   //exit();
13. }

I also moved the dispose() out of the drawing section.
Untested code...