Document Download without user action

Depends on where do you want the document? If you need it in a process then call integration to get document from s3 in process model. 
if you want to show in UI then get document from s3 to appian folder first then show to user using documentViewer.

If you don't want to store documents in Appian and just want to access them from S3 and display them in Appian,
then use the Apryse WebViewer Plugin to display S3 documents directly in Appian without storing them. Install the plugin, configure CORS settings on your S3 bucket to allow Appian domain access, then display in your interface. This is the only reliable solution since native Appian components require documents to be stored locally first.

If you want to store the document in Appian and use native Appian functionality to display it, use the below approach,
Use the Amazon S3 Utilities Plugin - configure AWS credentials, create a Process Model with S3 Download smart service, and set up automatic triggers (timer/web API) to download documents directly to Appian folders. Once the document is in Appian, automatically display it to users using the Document Viewer Component with a!documentViewerField() in your interface, passing the document ID from the download process - this will show the document immediately when the interface loads without requiring any user clicks or interaction.
For enhanced viewing with editing capabilities, alternatively use the Apryse WebViewer Component Plugin which provides richer document viewing.

I am assuming on user action you need to download the document into your system. Currently, there is no way to do that without an user action. 

Alternatively, if you want to save the document into Appian, pass the url as input to an integration object which will help in downloading the document into Appian. Choose method as GET and keep response body parsing as convert binary to appian document.