Thank you for the interest in RadPdfViewer.
The PDF content is not separated on text, graphics, highlights, etc. It consists of a flat single layer content that overlaps. This means that you cannot inset custom layer between the text and the background. The two options you have are to insert a layer below the content (as shown in the demo, then the PDF content will be placed above that layer's content), or above (the layer's content will be placed above the PDF content).
In your case you could try to insert the custom layer above the content, and make the highlights transparent. This will make both content and highlights visible.
Hope this answers your question.
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