There is no fixed limit, but Gramps could slow down with very large trees. Some of our users have trees with 20,000 people with no problem.
We don’t have a real-time collaborative web front-end, but you can generate static web reports suitable for hosting on a web site. [Update: Gramps Web collaborative front-end initially released July 2021]
Yes, PDF is a supported output format. What type of report design you are looking for?