Add support for shapes, and especially for shapes with textual content. 
In OOXML, shapes are represented by the wps:wsp element, and shapes with textual content by <wps:txbx>, <w:txbxContent>.
Such shape can be added to a Word document using the Insert -> Text -> Text Box -> Draw Text Box, or through a shape's context menu -> Add Text.

Do not confuse these shapes with Text/Rich Text content controls (see https://feedback.telerik.com/Project/184/Feedback/Details/190057 )

With the current implementation of the RadFlowDocumentEditor`s CharacterFormatting.FontWeight.LocalValue accepts only FontWeights.Bold and FontWeights.Normal and throws an exception when setting different weights (ex. Black, Thin, etc.).

Introduce Run.IsHidden property or similar, which hides the text in the document.

In MS Word, text is hidden using the Home -> Font -> Font -> Effects -> Hidden. In the Rtf format, such text is preceded (marked) with a '\v' tag. In OOXML, the tag is <vanish/>. The display: none; style from HTML is mapped to hidden text when document is converted to DOCX.

Hidden text is visualized with dotted underline when formatting symbols are shown. Hidden text is not exported to PDF.

Is there a way to prevent font embedding in the RadFlow methodology or is utilizing RadFixed the only way to have a reasonably sized PDF document?

When some character is not supported by the font, the fallback mechanism should try finding some other font that is capable of rendering the unsupported character. However, RadPdfProcessing fallback mechanism does not always find the correct font which sometimes result in wrong glyph visualization or in missing glyph.

Workaround: Font that supports these special characters may be used. This way the fallback mechanism will not be needed to export the PDF text.

When a paragraph is placed inside a list item (<li> tag), the paragraph properties are not applied to it on import.

When text content (text in paragraph, text in span) in HTML contains line break (\r, \n, or \r\n), it should be imported as space. Instead, the new lines are currently removed. 

For example, <p>first\nsecond</p> (line feed between the words) is imported as run with content "firstsecond" instead of "first second".

When a document has a list in it and the element containing it is cloned into another document, there is a key not found exception if you try to export the second document.

            RadFlowDocument document = new RadFlowDocument();
            if (tableDocument.Sections.First().Blocks.First() is Table workSheetTable)
            {
                Table clonedTable = workSheetTable.Clone(document);
                var newSection = document.Sections.AddSection();
                newSection.Blocks.Add(clonedTable);
            }

            byte[] pdfBytes = pdfProvider.Export(document);
            byte[] docBytes = docxProvider.Export(document);