RE:How to download Document link .

You can download a file AFTER the upload form has been submitted. Then use this https://docs.appian.com/suite/help/21.3/Document_Link_Component.html

Do you have a more specific question?

you cannot create a download link in a expression rule.
A download link is part of an Interface.
 There is a certain number of "interface components" which are just working within a SAIL Form/Interface context,

-> check for "interface components" - you can find a!documentDownloadLink() in the "Action" subsection.
https://docs.appian.com/suite/help/21.3/Appian_Functions.html