The PDF backend ignores the cosmetic attributes for certain pens.

The attribute indicating, if a pen is cosmetic or not gets stored in QPdf::Stroker ( see qpdf.cpp line 655 ). But later in QPdfEngine::drawPath the stroker is not used, when the pen is regarded as "simple".

According to qpdf.cpp line 1113 a pen is "simple", when the following conditions are met:

(penBrush.style() == Qt::SolidPattern) && penBrush.isOpaque() && d->opacity == 1.0

According to the code it is possible to get the cosmetic attribute being respected by setting the penBrush to something non opaque. And indeed setting a pen color with an alpha value of 254 worked in our tests.

This bug also appears in earlier versions, maybe since the code was implemented.