When using 'Print to PDF' to export an experiment that contains a chemical drawing with a .png image included, the reaction will be rendered without the image in the PDF.
Solution: This is the expected behavior of the application, currently the rendering of images inside chemical drawing elements is not a supported feature. As a workaround select the option 'Embed documents as attachments' when printing to include the .cdxml file with the image as an attachment in the PDF document.