Without programming, if you have an anchor in HTML code with
pointing to document's URI, you can simply use content menu item "Save As":
<a href="myDocument.pdf">Download document</a>
If the use clicks the link, it might be just loaded as a page, depending of file type.
This is how you can enforce downloading with "Save As" dialog using
To upload a file, you just need a Web form with method "POST", but to process the upload you need some server-side coding.
This is the detailed ASP.NET tutorial: http://www.asp.net/data-access/tutorials/uploading-files-cs
This CodeProject article provides very clear and simple explanation: File Upload with ASP.NET