Currently, NotSupportedException is thrown when trying to import a document that uses Type 3 fonts.
Update: With Release R1 2021 the import of Type 3 fonts is already developed but the export is still not supported.

The hierarchical document structure is a means to describe the PDF document structure, which is currently not supported. See 10.6.1 Structure Hierarchy on 856 page of the PDF specification for details.
This document structure is an alternative to the standard PDF structure and most non-Adobe software does not support it.

The document might have been created with Adobe software like Adobe LiveCycle Designer. Very often the document contains some fallback text in the standard PDF structure like:
"Please wait... If this message is not eventually replaced by the proper contents of the document, your PDF viewer may not be able to display this type of document."
"The document you are trying to load requires Adobe Reader 8 or higher. You may not have the Adobe Reader installed or your viewing environment may not be properly configured to use Adobe Reader. For information on how to install Adobe Reader and configure your viewing environment please see  http://www.adobe.com/go/pdf_forms_configure."
"For the best experience, open this PDF portfolio in Acrobat X, Reader X, or later."

RadPdfProcessing currently supports interactive forms whose data is defined directly in the document. Add support for interactive forms based on the Adobe XML Forms Architecture (XFA). The entry is defined inside the interactive forms dictionary and refers to an XML stream containing the information of the form. More information is available on page 722 from Pdf Reference, version 1.7.
Updates:
November 2024
From PDF 2.0 (ISO 32000-2) the XFA forms are depreciated. For this reason, we are changing our plans and declining this item, but we will provide import-export support to avoid losing existing XFA content. Please, subscribe to the following public item: PdfProcessing: Add support for XFA fields import-export.

In this case, we should use a default alpha source value which is the opacity source.

Currently, when registering *.otf font file with FontsRepository.RegisterFont method an exception is thrown during the font creation. 

WORKAROUND:  The font file may be converted to TTF format (*.ttf) which is successfully registered. This conversion may be achieved with FontForge by opening the font file and then choosing File -> Generate Fonts -> TrueType -> Generate. More information on FontForge program may be found here: https://fontforge.github.io/en-US/

This exception seems to be related to AES algorithm PaddingMode property. 

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

Add support for a strikethrough property or strikethrough-related properties in the TextProperties class.

Automatically.PNG
Please find the attachment.
Sending the info on spelling error

The Matrix property seems to be incorrectly interpreted during the import process.

For some documents, the clippings and colors of the geometries are modified on an import-export scenario.

This includes loading form fields data from FDF file and saving the form fields data to FDF file.

Stencil color has to be preserved for images with ImageSource when ImageMask property set to true. Not preserving it results in wrong colors (for instance the image may be visualized as a black rectangle).

Available in R1 2019 Official Release Version.

Currently, we support inserting png, jpg and other raster graphic images. Provide a way to insert image from SVG (vector graphic image format), by creating a FormSource from its parsed XML.

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.

Although this is not a valid postscript scenario we should not throw exceptions and we should instead skip the invalid dictionary end characters.

Fix available in LIB Version 2018.1.312.

When merging, only the widgets are copied but not their fields. This can lead to ArgumentNullException when exporting the document. Implement copying the fields and consider cases which include collision of names of the fields.

Workaround:
Repair the generated document using the following code:
foreach (Widget widget in this.pdfDocument.Annotations.Where(annot => annot.Type == AnnotationType.Widget))
{
    if (!this.pdfDocument.AcroForm.FormFields.Contains(widget.Field.Name))
    {
        this.pdfDocument.AcroForm.FormFields.Add(widget.Field);
    }
}
In case of Name collision, you can use the attached project or the attached project to the following feedback item to rename the duplicate fields: Provide a way to rename FormFields.

A NullReferenceException is thrown when trying to clone the Signature property of the field.

Available in LIB Version 2018.1.312.

Trying to clone the Signature of a SignatureField leads to InvalidOperationException as the FieldName of the cloned signature is already set.
Workaround: Remove the signatures before the merging of the document:
private static void RemoveSignatures(RadFixedDocument document)
{
	List<FormField> signatures = document.AcroForm.FormFields.Where(ff => ff.FieldType == FormFieldType.Signature).ToList();
	if (signatures.Count > 0)
	{
		foreach (FormField signature in signatures)
		{
			document.AcroForm.FormFields.Remove(signature);
		}
	}

	List<SignatureWidget> signatureWidgets = document.Annotations.Where(a => a.Type == AnnotationType.Widget && a.GetType() == typeof(SignatureWidget)).Cast<SignatureWidget>().ToList();
	if (signatureWidgets.Count > 0)
	{
		foreach (var signatureWidget in signatureWidgets)
		{
			foreach (RadFixedPage page in document.Pages)
			{
				if (page.Annotations.Contains(signatureWidget))
				{
					page.Annotations.Remove(signatureWidget);
				}
			}
		}
	}
}

In business or enterprises there is more and more demand for PDF/A-3.

In Germany it is Zugferd for hybrid invoices, in Brazil Nota Fiscal Eletronica 2.0, in France FACTUR-X.

Also for longterm preservation it is import to support PDF/A-3.

If I would be the PM of this library this would be my number one priority!