Hi Christof, Mendix can generate documents. I would recommend checking out the new 'Document Generation for Mendix'. This module is superior to PDF's from document templates.
For the performance there is no clear cut answer but:
- Mendix can be scaled up to levels where major companies have high performing apps running on it
- The TaskQueue is a good way to handle load on the server
- Mendix Cloud is scalable per instance and also over multiple instances
You can also try to schedule generation of documents in times the server is less used by Scheduled Events.
Good Luck,
Peter