You can share any document or any entity (Task, Feature, Project, etc). Sharing is public and link never expires.
To share a document, find Share icon in the top right corner and generate a link:

When you share a document, all mentioned entities are shared as well. For example, if you have a list of Tasks in a document, these tasks will be accessible.
To share an entity, find Share icon in the top right corner and generate a link:
Entity is shared with all linked collection. For example, here we have an Objective with many related entities: Key Results, Ideas, Features, etc. When you share this Objective, all entities in collections are accessible:

Sharing works two-level deep. It means that linked collections will be shared, but no deeper. For example, if you have Objective - Feature - Task hierarchy and share an Objective, then Tasks will be unaccessible in a shared Objective.
NOTES:
So far you can't specify what is shared. Please ping us via Intercom if you need to hide something and we will add this into future scope
So far it is not possible to collaborate in shared documents and entities