You need to explicitly set the correct MIME type when embedding the file into the PDF. This is especially important for standards like PDF/A-3 and Factur-X, which require strict metadata and MIME type declarations for embedded files.

The default value /application/octet-stream is too generic and not compliant for XML attachments.

 

Documents can be signed using the end certificate of a certificate chain, instead of the whole chain. This improves the performance of the signing.

In version 1 there are three profiles that have different levels of detail in their structure: Basic (B), Comfort (C) and Extended (X): https://mind-forms.de/e-rechnung/welches-zugferd-profil-kann-ich-verwenden/ 

Add support for encrypting documents with an AES-128 algorithm.

Add support for the PAdES-BES signature standard

Add support for /TR (Transfer Function) property of SMask.

When importing a document with an invalid creation or modification date, an exception is thrown:

• FormatException: 'The input string '' was not in a correct format.'
• ArgumentOutOfRangeException: 'Year, Month, and Day parameters describe an unrepresentable DateTime.'

From the Part 4: PAdES Long Term - PAdES-LTV Profile (ETSI TS 102 778-4) Specification: 

"Validation of an electronic signature requires data to validate the signature such as CA certificates, Certificate Revocation List (CRLs) or Certificate status information (OCSP) commonly provided by an online service (referred to in the present document as validation data). If the document is stored and the signatures are to be verifiable long after first created, in particular after the signing certificate has expired, the original validation data may no longer available or there may uncertainty as to what validation data was used when the document was first verified."

When writing a RadFixedPage containing widgets using the PdfPageStreamWriter.WriteContent() method an exception is thrown.

The exception: System.ArgumentNullException: 'Value cannot be null. Parameter name: context'

Provide an API for externally signing the document hash.

Add support for ShadingType 1.

Currently, you cannot sort filtered cell ranges that contain merged cells. This is supported in Excel.

Handle the import of documents with an invalid cross-reference stream offset.

XlsFormatProvider: Add support for hidden rows.

Add support for Fields when exporting to TXT format. When a field is exported to *.txt, only the result should be exported (as in HTML). In the current implementation, the field characters, field code, and field result are all exported.

Optimize PDFs for quick viewing on the web, especially for mobile clients. Linearization allows your end users to view large PDF documents incrementally so that they can view pages much faster in lower bandwidth conditions: https://developer.adobe.com/document-services/docs/overview/pdf-services-api/howtos/linearize-pdf/ 

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."

Add support for Ink annotations (InkML).

Currently, PdfProcessing provides support for exporting images in .NET Standard: https://docs.telerik.com/devtools/document-processing/libraries/radpdfprocessing/formats-and-conversion/convert-to-image/using-image-format-provider 

Add support for the FILTER function.