The redaction annotation is described in the following PDF reference document: http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_reference_addendum_redaction.pdf You may also see sample document with this annotation type attached. Since LIB Version 2017.3.925 annotations of type Redact can be visualized.
The document will be imported with a loss of content. The reason is that ASCII85Decoder will throw an ArgumentException when the stream doesn't end with '~>'. Although this is invalid case according to PDF specification, other applications are handling it and the document is successfully visualized in them. We may also consider implementing workaround that handles such invalid streams.