What is the app you are open your PDF with?
Only Adobe Reader on Mac or PC or Acrobat Pro should qualify when "show overprint" is set "on" in the preferences.
Do not trust any other PDF reading software.
I'm using Preview on Mac OS to view the PDF. It's possible that my clients could be using any software to read the PDF so it would be ideal to have this work in more than just Adobe Reader.
hm, then you have to do a harsh decision, your only choice to be on the "safe" side:
Export to PDF v1.3 (Compatibility Acrobat 4 (PDF 1.3) where you tick on "Simulate Overprint" in the fourth tab of the PDF Export settings.
This workflow is not recommended for PDFs you want to print:
Just for screen viewing!
Then all "overprint" and "transparency" is baked into the PDF so to speak.
Here a commented screenshot from my German OS:
Danke schön Uwe! That fixed the problem. Unfortunately we will also need to print these later so I will keep my old settings and use the PNG anyway. Your help is much appreciated.