Can the plugin offer me this?

Hi,

Thanks for contacting us here about this extension.
As I guess, you are talking about WP File download plugin.
These features are impossible with our WP Media Folder, so I moved this to the right category.

I need to store revisions of documents

This impossible to edit the documents directly from our plugin. You can update a file by uploading its new version.
For more information: https://www.joomunited.com/documentation/wp-file-download-documentation#toc-2-6-update-a-file

I need to categorise the documents

You can order by the file format in backend.

I need to only publish a revision or new document when a specific user has approved the document after they have been emailed to say they need to approve it.

This feature is not possible now. The email will be sent if a new file is uploaded.

I need to be able to search with the content of the documents (Word, Excell, PDF)

Yes, you can search the content in your documents by enabling option: "Plain text search".
Please go here for more details: https://www.joomunited.com/documentation/wp-file-download-documentation#toc-4-2-search-engine

Hope it helps!
Cheers,