Comment 14 for bug 1859632

How about this breakdown? Boils down to the same root causes a bit...

Okular #1: Stamp Annotations are not printable
(I can confirm this, for both standard and non-standard icon names)
Root cause: PDF print requests are forwarded from Okular to the Poppler generator (see PDFGenerator::print). But Poppler can't render stamps for two reasons:
-Poppler doesn't implement default appearance streams for standard icon names, so it can't render stamps like "Draft" and "Approved". See [1], "I leave the bug open becuase there are still annotations missing default appearance streams (Free Text annots, rubber stamp, ...)".
-Poppler can't generate/render/save an appearance streams for your custom *.svg, *.png, whatever image yet. Displaying them in Okular on screen only works because Okular renders stamps on its own.

Okular #2: Custom stamp annotations (i.e. non standard icon name) are not visible in other readers
Root cause: If stamp is created by Okular+Poppler, no appearance stream is generated/embedded into the PDF. Other readers should implement a default appearance stream for standard icon names, but for non-standard icon names like "kde", "okular", <path_to_local_file> they can't know what to display. Maybe of interest is this pdf.js (firefox) issue [2]: "Most annotations, even unsupported ones, render just fine in PDF.js because of their appearance stream. However, there exist PDF files with annotations that do not have an appearance stream (even though this is a deprecated practice). In the latter case, PDF.js displays nothing.". There have been some thoughts about handling of appearance streams in Poppler [3], but I don't know the current state and desire there.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=23108#c21
[2] https://github.com/mozilla/pdf.js/issues/6810
[3] https://lists.freedesktop.org/archives/poppler/2009-June/004789.html