Not 100% sure what we are trying to prove, seems the objective is a pdf that will print "as seen in Gimp 3.0" Not easy for me, especially since I do not have Adobe Acrobat / Affinity Photo ......
A pdf from Gimp 3.0 is PDF-1.7 using a cairo library. An old solution is before exporting to PDF is make a new top layer using Layer -> New from visible. That keeps your layers for future editing (as a Gimp .xcf)
Differences can be seen - left to right Gimp 3.0 <> LibreOffice <> Scribus 1.7
and now the PDF with the new-from-visible
...might be worth a try.
EDIT: or maybe not, just checked in linux
and it shows as having transparent sections,
A flattened image with alpha channel removed looks better but still has a "Soft Mask"
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A jpeg export then converted with ImageMagick
magick in.jpg out.pdf
Has no metadata at all - no transparency
A pdf from Gimp 3.0 is PDF-1.7 using a cairo library. An old solution is before exporting to PDF is make a new top layer using Layer -> New from visible. That keeps your layers for future editing (as a Gimp .xcf)
Differences can be seen - left to right Gimp 3.0 <> LibreOffice <> Scribus 1.7
and now the PDF with the new-from-visible
...might be worth a try.
EDIT: or maybe not, just checked in linux
Code:
grep -aE -e '/[Cc][Aa] +0?\.[0-9]' -e '/SMask' -e '/S /Transparency' filename.pdfA flattened image with alpha channel removed looks better but still has a "Soft Mask"
-----
A jpeg export then converted with ImageMagick
magick in.jpg out.pdf
Has no metadata at all - no transparency