Here's what I have in mind:
- Individual logins limited to particular client account. No need for deeper authorization past that. Account, has_many Users
- Table of contents listing related documents, with last-updated timestamp
- Ability to download (styled) PDF version of the current document state
- Full-text Search
- Email notifications for account creation, document creation and document update
- Branding per account and overall (logo, colors)
Nice to have:
- Markdown support for docs
- Comments on documents
- Versioning
- Ability to download zip of entire document set of PDFs
The key thing is: That's it. No project-management features, no calendar.