According to the PDF Specification: A redaction annotation (PDF 1.7) identifies content that is intended to be removed from the document.

There are two possible options to workaround this functionality:

1. To iterate the document content and replace the matches which meet certain conditions with an empty string:

   foreach (RadFixedPage page in document.Pages)
   {
   	foreach (ContentElementBase elementBase in page.Content)
   	{
   		if (elementBase is TextFragment textFragment)
   		{
   			if (IsValidEmail(textFragment.Text))
   			{
   				textFragment.Text = string.Empty;
   			}
   		}
   	}
   }

   Note: the desired content could be split into several text fragments and if so a custom parser should be created.
   
2. To iterate the document content and check if the TextFragment position is within a specific RectangleGeometry: 
   

   foreach (RadFixedPage page in document.Pages)
   {
   	foreach (ContentElementBase elementBase in page.Content)
   	{
   		if (elementBase is TextFragment textFragment)
   		{
   			if (IsIntersecting(rectangleGeometry, textFragment))
   			{
   				textFragment.Text = string.Empty;
   			}
   		}
   	}
   }

   Note: it should be taken into account that the text fragment could flow out of the rectangle.