When importing a document containing Outlines (Bookmarks) containing Actions with an empty dictionary:

<</Title(Bookmarks)/Parent .../First .../Last .../Prev .../A<<>>/Count ...>

leads to an exception thrown: System.MissingMethodException: 'Cannot create an abstract class.'

From the PDF specification:

Prev: (Present only if the file has more than one cross-reference section; must not be an indirect reference) The byte offset from the beginning of the file to the beginning of the previous cross-reference section. 

When importing such a document an exception is thrown: System.ArgumentNullException: 'Value cannot be null.

When extracting text from a composite font the ToUnicode mapping should be used but when this mapping is missing the char codes must be mapped by constructing a secondary map from fonts ordering and registry. 

A possible workaround could be to register a TrueType font using the FontsRepository.RegisterFont() method:

FontsRepository.RegisterFont(new FontFamily(fontName), FontStyles.Normal, FontWeights.Normal, File.ReadAllBytes(pathToFont));

FontsRepository.TryCreateFont(new FontFamily(fontName), FontStyles.Normal, FontWeights.Normal, out FontBase font);

foreach (RadFixedPage page in document.Pages)
{
	foreach (ContentElementBase contentElement in page.Content)
	{
		if (contentElement is TextFragment textFragment)
		{
			if (textFragment.Font.Name == fontName)
			{
				textFragment.Font = font;
			}
		}
	}
}

When importing a PDF document, opening the stream in using statement, and using the default constructor of the PdfDocumentSource class, an exception is thrown:

using (Stream stream = ofd.OpenFile())
{
	this.pdfViewer.DocumentSource = new PdfDocumentSource(stream);
}

using (Stream stream = ofd.OpenFile())
{
	PdfImportSettings importSettings = new PdfImportSettings
	{
		CopyStream = true
	};

	this.pdfViewer.DocumentSource = new PdfDocumentSource(stream, importSettings);
}