One of our clients had asked for a very sophisticated library to support their company's legal communication review process. All the audit information is being kept in a version history of a document. The problem started when they started editing PDF document by applying the "Redaction" tool of Acrobat, by design you cannot save the original document, you can only save the edited document using "Save as".
When Redaction is applied to the document, the document is being stripped of all original information and metadata to protect the document’s original information. In order to achieve this level of document protection, the document must be saved as a new document which removes the original document from the SharePoint library and saves the “redacted” document as a new entity.
As soon as the document had been "saved as" within the SP library it gets a new item ID associated with it, this way preventing any already running workflows to be completed and removes all original metadata information. If you try to complete a workflow task associated with this "Redacted" document, it will give this error: "The data source control failed to execute the update command." because the original item associated with this workflow does not exist anymore.
I recommend that you do not to use redaction tool at all within a SharePoint library, resort to the use of “revision” tool, or don't expect this document to have anything in common with the original document.
By the way, the same applies to any Microsoft Office document which is "saved as" even if it is saved over the original doc by replacing it, but at least within Office you have to intentionally choose to "Save as". But with adobe "redaction" tool you are forced into "save as".
I guess people have to be educated on the purpose of "redaction" feature, if it was behaving any differently, it would completely defeat the purpose of “Redaction” tool.