The Description label comes from a resource file so it is localizable but not customizable. The original purpose of the Description field was so that one could copy the text of the pdf into the description and then hide the description. It was a solution for the problem that we don't have a good way to index the content of the pdf files into the search index, so by putting the text in the description it gave us an easier way to index the content of pdf files so they can be found using site search.
I looked at the code about the download and I've put an item in our project tracker to add a module setting so that you can make it download pdf files as an attachment instead of opening it in the browser. It won't be difficult so I will probably get to this for the next release. It will be a module level setting not a per file setting though.
Best,
Joe