GIMP PDFs have transparency

[waltonm]

I am using GIMP 3.0.0 on Windows 10 system

I made a PDF (Export as PDF) from a multilayered file. It had transparency (Acrobat > Preflight > List transparent objects). I flattened the file, exported as PDF, and it had transparency.  I exported the flattened file to JPG, then exported the JPG to PDF, and it had transparency!

To further focus in on the potential solution or problem, I:

1. created a JPG in XnView
   
2. opened it in GIMP, then exported it as a JPG
   
3. closed the file, then opened the GIMP produced JPG
   
4. exported it to PDF, using default settings
   
5. checked the transparency in Acrobat: "Transparency used." (A, in attachment)
   

 

To test this, I repeated the steps using Affinity Photo, I:

1. used the JPG from XnView
   
2. opened it in Affinity Photo, then exported it as a JPG
   
3. closed the file, then re-opened the Affinity Photo produced JPG
   
4. exported it to PDF/X-4, setting it to permit transparency/layers, if there is any
   
5. checked the transparency in Acrobat: "No problems found." (B in attachment)
   

I have repeated this in GIMP using Print > Print to doPDF, cutePDF, AdobePDF, all with no transparency. 

Some printers will reject PDFs with transparency.  I am trying to produce a PDF from GIMP with no transparency and no other programs (like Adobe PDF).  Is that possible?

Walton