Adding an option to set the tranparency of the `Freehand line` tool would provide a way to use it to highlight text in PDFs whose text isn't recognized and thus not supported by the `Highlighter` tool. Currently the freehand line is opaque and makes the text unreadble. Workarounds with small lines or others are possible. Reproducible: Always
This is possible on Okular already, see https://docs.kde.org/stable/en/kdegraphics/okular/configannotations.html You can even add a new "HighlighterfornotrecognizedText": Add a new annotation of type "Straight Line", you favorite color and opacity < 100% Invalid report imho
What Burkhard says, you can already do that