You can convert the document to PDF and use the approach bellow:

Currently, silent async printing may be achieved by using RadPdfViewer WPF control. Sample demo showing how to achieve this may be seen at the following forum post:
http://www.telerik.com/forums/pdfviewer-print-makes-ui-unresponsive#js0YdzFWc0Oa8C3g6a18lg

The RadPdfViewer WPF control used does not need to be displayed, making this demo a good workaround for ASP.NET AJAX clients.

Implement functionality allowing to specify that when a table is split on multiple pages, a specific row should be shown on each page.

Workaround: in the attached demo projects.

To bring RadPDFProcessing to the next level you should support XMP metadata. This would extremely help to use pdf files from RadPDFProcessing in digital workflows.

Reference: http://www.adobe.com/devnet/xmp.html

Support conversion of XPS documents to PDF using RadPdfProcessing.

Currently, similar scenario may be achieved by converting Tiff images to BitmapSource and inserting them to RadFixedPage as images by using FixedContentEditor.
Note: A sample approach how to achieve such functionality is demonstrated in the Converting Multi-page TIFF Images to PDF KB article.

Currently, PdfStreamWriter writes only PDF pages.

If we implement API for merging multiple pages or whole documents we would be able to preserve links between pages (which are currently being corrupted).

Merging whole documents would also allow preserving document properties (bookmarks, interactive forms, ...). For a more detailed list of the unsupported document properties you can refer to our documentation: https://docs.telerik.com/devtools/document-processing/libraries/radpdfprocessing/formats-and-conversion/pdf/pdfstreamwriter/features.

We may also implement API for adding initial document properties from some existing PDF file.

Add ability to create, import and modify layers (optional content) in a PDF document.

Currently RadFixedDocumentInfo is used only for export of Author, Title and Description document metadata properties. This should be extended to support custom properties. We should also implement the import of RadFixedDocumentInfo.

or there should be an optional parameter "bool AcceptNewlines" or "bool IgnoreNewlines"

Currently, this can be implemented like this:

private void InsertTextWithNewlines(Block block, string text)
{
    string[] lines = text.Split(new[] {"\r\n", "\r", "\n"}, StringSplitOptions.None);

    for (int i = 0; i < lines.Length; i++)
    {
       block.InsertText(lines[i]);
       if (i < lines.Length - 1)
       {
          block.InsertLineBreak();
       }
    }
}

When merging documents containing the same embedded font every time a document is merged the same font is repeatedly embedded in the merged document. This leads to very large files.

A possible workaround before this feature is developed could be to use the PdfProcessing model to import and merge the documents. After the documents are merged to iterate the document`s content in order to cache the fonts using their name and if there are duplicated names to set the already cached one (check the attached project). 

As a side note, the suggested workaround doesn't cover all possible cases (e.g. if there are fonts with a common name but different font sets (modified fonts or font subsets) this may lead to missing/different characters in the produced document). So in order to use this workaround, you will need to ensure the font names are unique for every different font.

Text fields should support rich text strings allowing the user to use rich text elements and attributes.

As one can use annotations we should support attachments per page or per pdf-file.

Square annotation displays a rectangle on the page. When opened, it displays a pop-up window containing the text of the associated note.

When creating a GradientStop, an alpha channel of the color could be applied. However, this setting is not respected it the result document and the gradient is with full opacity.

Steps to reproduce:
1. Create a gradient with an alpha channel:

RadFixedDocument document = new RadFixedDocument();
RadFixedPage page = document.Pages.AddPage();
FixedContentEditor containerEditor = new FixedContentEditor(page);

LinearGradient linearGradient = new LinearGradient(new Point(0, 0), new Point(30, 30));
linearGradient.GradientStops.Add(new GradientStop(new RgbColor(10, 0, 207, 0), 0));
linearGradient.GradientStops.Add(new GradientStop(new RgbColor(10, 0, 102, 204), 1));

containerEditor.GraphicProperties.FillColor = linearGradient;
containerEditor.DrawRectangle(new Rect(10, 10, 48, 29));

2. Export the document to PDF

Observed: The alpha channel is not respected and the gradient is with full opacity