Partially transparent images lead to artifacts
To reproduce this problem, take some PDF file and then use Xournal to add a PNG file with alpha channel. That can result in a file like this: dummy-a.pdf.
When opening this file in a poppler-based viewer such as Okular or Evince, it shows a border around the opaque part of the embedded image:
I first thought this was a Xournal bug, but the developer says they are not adding any kind of border, and indeed Firefox shows no such artifact when opening the same PDF file:
I am no PDF expert, but the Xournal developer says this might indicate a sub-pixel misalignment of the "soft mask" associated with the embedded image, and maybe there's something poppler can do to render these files better, the way Firefox does.
Okular says "Using Poppler 20.09.0".