How do I make files downloadable for a particular role in Plone?
The codeless way to do this is to make use of Plone's workflow system.
Out-of-the-box, Plone's file and image content types do not have their own workflow. That means that files and images will simply inherit the publication state of their parent folder. This is easy and sensible, but it doesn't meet the need you're describing.
To change the situation, you may use the "types" configuration panel to turn on independent workflow for files and images. Then, their publication status may be set separately from their containing folders. Typically, you'd choose the same workflow that you're using for documents. Then, you may publish a folder and list its contents while having the files within be private -- thus requiring login for viewing.
If you need this to work differently in different places, you may turn on "placeful" workflow (turn it on by adding it in the add-ons panel; it's pre-installed, but not active). This allows different workflows in different parts of a site. It increases complexity, but is often an ideal solution to this kind of puzzle.
I wish to make the contents of a folder in Plone downloadable only for certain roles. Can this be done easily? At present anybody who clicks the hyperlink for file name in the folder contents can download the file easily. I know about the site-wide option of overriding the at_download code using ZMI.