It's not possible to attach documents to the system's tasks. Alternatively, sending documents to managers after a new record is created, can be automated using Ripples.
Create the query
Click Access Button
then click HR Admin.
Click Queries, then click on the plus + button.
Select Employees Details then select Start Date.
Click Next then on start date filter for Today.
Click Next then give it a name and click Save.
Create the ripple
Click Access Button
Go to Settings and click Ripple.
Click the plus + icon.
Select Ripple type as Background.
Then the status as Active.
Choose the Employee/Manager Access as None.
Add Step.
Step Type: Email.
Click Next.
Name the step, fill the subject and the body of the email.
Click on Next.
Upload your document by clicking Click Here.
Click Next.
Select who will get the email.
To Address: Manager.
Click Save.
Click Save.
Link the query with the ripple
Click Access Button
Go to Settings and select Ripples.
Click on the Star icon next to the ripple, then click on Add New Item.
Give it a name, then select the query you created.
Select the ripple you created.
Set the value to 1 in [This ripple will never repeat for an employee unless you allow this ripple to repeat every x days] field.
Click Save.
📌Note: You're able to use Mail Merge feature to upload document template to users in bulk.