Generate a link using below format:
http://spsite/subsite/_layouts/download.aspx?SourceUrl=http://spsite/subsite/shared documents/file.docx
And use this URL in Anchor tag / window.open etc. as per
the requirement.
Hope that helps.
A Blog about SharePoint Server 2010, Management Shell (Powershell), SharePoint Designer 2010.
using Microsoft.SharePoint.Utilities;