There are three options to trigger Rules and apply Actions using Automation 'On Creation', 'On Edit' and 'On all Tasks Completed'.
Fig 1 - Trigger Options
The 'On Creation' trigger will apply your rule when you create specific new records. Such as Adding a brand New Contract record, or creating a new Contract Amendment or new Contract Document record.
So your Rule could be:
'On creation' of a <Contract Document>, create a <Task> for the <Account Manager>.
If you do not add any Criteria to your Rule then it will apply to all of your Entity records, if you wish the Rules to apply in certain circumstances then you must specify when the rule should apply using the Criteria.