When creating or editing an Asset record, you can assign documentation it. These links can be viewed from the Documents tab of the record, or in the secondary view panel in the manager.
Most projects import the document/Asset associations. You will need to import the document record list and run a second import to link the assets to those document records.
A fast way to assign documents to assets, is through a Batch Editing. For example, if a P&ID was provided and all the asset tags were already in the database, a user can batch query and edit a collection of tags on the drawing an assign to the drawing in one step.