Hi FireDance,
>> How to make color transparent so that words can be seen after highlighting?
I assume you are using Drawing Line object at runtime. You can set the
Storke property of Line object to set its color. Use the following code snippet.
Line line = new Line();
line.Stroke = new SolidColorBrush(Colors.Transparent);
I have something more to mention, normally, we don’t use the transparent color for highlighting content. We change the background and content foreground color like the following image.

I would recommend you use the same way.
If it doesn’t help, please feel free to let me know. Post more information about your scenario, such as how you display the PDF or send me a repro project for reproduction.
Regards,
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