From R1 2017 all annotations (including sound and video) are supported for import-export scenarios by using PdfStreamWriter class. 

These feedback item will be opened as we may add API for generating and inserting new sound annotations from scratch in the file.

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.

Support conversion of XPS documents to PDF using RadPdfProcessing.

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

Encryption Algorithm 4 is implemented for R1 2018 official release. This includes all encryptions except AES 256 which is related to Encryption Algorithm 5. For AES 256 you may vote and follow this feedback item:
https://feedback.telerik.com/Project/184/Feedback/Details/240154-pdfprocessing-support-encryption-algorithm-5-with-aes-256