This is a very neat application and at the core of what the blockchain actually provides, non-repudiation. This is one of the killer-apps for blockchains. Well done.
Having said that for normal users this implementation is still way out of bound. The requirement to copy/paste a private key is a bridge to far for most users, regardless of the security risks of doing this. A direct (secure) integration with the client will need to be made to make this seamless for end-users and to succeed with wider adoption.
The step after that is seamless integration (e.g. an App) with Sharepoint (proprietary, may be difficult) or Nextcloud (open source) like cloud documents management systems.