A dictionary specifying any resources (such as fonts and images) required by the form XObject (see Section 3.7 from http://partners.adobe.com/public/developer/en/pdf/PDFReference.pdf

The current implementation relies on valid cross-reference table offsets in the PDF documents so that PDF objects are easily found and parsed. However, a mechanism for importing documents with invalid cross-reference table offsets may be implemented.

The attached project shows how to repair such documents.

Add UI for adding comments and ability to save them. Comments are represented in the document model as text annotations (annotations with Subtype Text). 

This is Unicode (UTF-16BE) encoding for the Adobe-GB1 character collection; contains mappings for all characters in the GB18030-2000 character set.

Described on page 443 in Pdf Reference 1.7.

RadPdfViewer partially supports CCITTFaxDecode (that is basically TIFF). There are some limitations that are not supported at this stage like EncodedByteAlign = true or Group 3, 2-D encoding.

Include support for layers

Add a navigation pane to view attachments under a pdf. it is possible to have various attachments under a pdf file, such has another pdf. For example, when converting an email with attachments, the resulting pdf has attachments. 

RadPdfViewer has no possibility to view those right now. Could be an event and let the calling program manage this.

For our needs, I would like an event, because our attached files are also pdfs, so I could open them in a new tab

The issue is reproducible only on Windows XP. On newer versions of windows, the images are rendered as expected.

WORKAROUND: You may try implementing custom filter that handles the CMYK scenario. How to implement custom DCTDecode filter is described in this documentation article:
https://docs.telerik.com/devtools/winforms/pdfviewer/customize-and-extensibility/customize-pdf-rendering

Overprinting is option in the GraphicState that allows combining colors when drawing one object above another object.  It is briefly described on page 284 and page 565 in PdfReference 1.7. However, the result of overprinting is device dependent and there are no strict instructions how the result should look like. This feature also includes support for the different blending modes.