The rendering of the image is not fully parsed and the next attribute cannot be properly read. 

System.InvalidCastException: 'Unable to cast object of type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfArray' to type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfInt'.'

When a ToUnicode CMap is defined for Simple Font (TrueType or Type1 font) which maps font character codes to two or more Unicode character values, the extracted text should contain the whole set of characters defined in the mappings. The current implementation of RadPdfViewer takes the first character specified in the ToUnicode mapping which leads to missing characters.

This exception seems to be related to AES algorithm PaddingMode property. 

When making changes to Form fields then flattening them and invoking the viewer`s Save method an exception is thrown: System.Collections.Generic.KeyNotFoundException: 'The given key was not present in the dictionary.'

Code to reproduce the issue:

this.pdfViewer.Document.AcroForm.FlattenFormFields();

using (Stream output = File.OpenWrite("Exported.pdf"))
{
	this.pdfViewer.Save(output);
}

this.pdfViewer.Document.AcroForm.FlattenFormFields();

using (Stream output = File.OpenWrite("Exported.pdf"))
{
	PdfFormatProvider provider = new PdfFormatProvider();
	provider.Export(this.pdfViewer.Document, output);
}

From the PDF Specification: "An ink annotation represents a freehand “scribble” composed of one or more disjoint paths. When opened, it displays a pop-up window containing the text of the associated note."

Sometimes the Ink annotations are imported as UnsupportedAnnotation and not displayed as expected. In such cases, the annotations could be removed from the RadFixedPage content in the following manner:

foreach (RadFixedPage page in document.Pages)
{
	List<Annotation> inkAnnotations = page.Annotations.Where(a => a.Type == AnnotationType.Ink).ToList();
	foreach (Annotation inkAnnotation in inkAnnotations)
	{
		page.Annotations.Remove(inkAnnotation);
	}
}