A user role must have the necessary rights to the <Documented - Visible on Payroll Portal Rights> screen section to control the visibility of a document.
If the user role has the correct rights then the Information Panel will indicate if the Document is going to be available to the External Payroll Portal User and allow the user to override visibility by clicking the Payroll Visible - Home and Payroll Visible - Host toggle buttons:
When the Payroll Visible - Home and/or Payroll Visible - Host toggles are toggled on, and if the payroll system matches the payroll system of the External Payroll Portal User, the document will then be available to be seen and downloaded by the Portal User in the Documents section:
Note that by default, the toggle buttons in the assignment documents are not available - the screen section right: Documents - Visible on Payroll Portal is required to enable them