The APEX developer GUI has an APEX application export function. The APEX installation zip includes a Java commandline export program called oracle.apex.APEXExport.
These enable one to export an APEX app as a SQL script - which in turn can either be imported via the APEX developer GUI, or run via SQL*Plus.
These exports excludes static contents the app may use on the web server.
I could be wrong, but my assumption is that you wish to download a copy of that app, which doesn't belong to you. Is that correct? It doesn't look like the app developer has provided a download option via the UI. Only the owner of that application, if they chose to do so, could provide you with an export of that app.