Inserting PNG ImageSource into a document and applying the document instance directly to the RadPdfViewer`s Document property (without exporting to stream) leads to a wrong image rendering.
Workaround: Export the document with the PdfFormatProvider:
MemoryStream ms = new MemoryStream();
PdfFormatProvider pdfFormatProvider = new PdfFormatProvider();
pdfFormatProvider.Export(radFixedDocument, ms);
PdfDocumentSource source = new PdfDocumentSource(ms, pdfViewer.DefaultImportSettings);
this.pdfViewer.DocumentSource = source;