i link: a!documentDownloadLink(document: fv!item.appianDocId ),
used the above two lines to download a document but it does not download , it open a tab with a message error : { "error": "APNX-1-0000-000", "message": "Invalid Content ID", "title": "An Error Has Occurred"}
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It seems the object you are trying to download does not exists in appian. So before passing the documentID to a!documentDownloadLink() function better to check the exitance of document in Appian using following functions:
isobjectexists returns true if object exists in Appian.
isobjectexists( "Document", /*documentID*/)
getcontentobjectdetailsbyid will return all the content of the object.
getcontentobjectdetailsbyid( /*documentID*/)