Is there a way to load this RadFixedDocument into a RadPdfViewer without writing the document to a file first and then loading it in the viewer?

Workaround:

You can achieve the desired functionality by exporting the RadFixedDocument to a MemoryStream instead of a file in the file system and then load it in the PdfViewer: 

RadFixedDocument document = CreateRadFixedDocument();

PdfFormatProvider pdfFormatProvider = new PdfFormatProvider();
Stream ms = new MemoryStream();
pdfFormatProvider.Export(document, ms);

radPdfViewer1.LoadDocument(ms);