Once again, please, accept my apologies for the delay. The reason behind the red color is that, roughly said, this color is present in the PDF file, but there is a functionality which overrides it and which is currently not implemented in RadPdfViewer. It is already present in our backlog and you can find the link to the public item here: PdfViewer: Add support for Transparency Group XObjects
If you'd like to, you can upvote and subscribe to the item, which will both increase its priority and notify you about any development.
As for a workaround, the only one that is applicable would be to edit the original file, more specifically the red background that is present in it. Whether this is feasible for your case depends on the specifics of your scenario, of course.
Please, let me know if you have any further questions or comments.
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