Joomla! 3 has native tagging features built in.
See the Joomla! documentation How To Use Content Tags in Joomla!
One way to accomplish your goals is to create a seperate article (i.e. a web page) with a brief synopsis for each downloadable paper, then use Joomla!'s built-in tagging capabilities to tag the actual page.
It can be that simple.
Keep in mind also that Joomla! has 'Smart Search' capabilities - so if you use the method described above, you are giving site visitors a number of intuitive ways to search for the 'downloadable papers' that they are looking for - built right in to Joomla! 3.
NOTE: Your question as posted does not take into account 'restricting access' to downloadable files. As long as you are not restricting access to the downloadable files (i.e. as some membership based sites do), you do not need to worry about using a 3rd party extension.
However, keep in mind that if using the method described above (i.e. publish an article with synopsis for each document, provide a download link, tag article), then know that anybody with a link to the downloadable file will be able to download it (without even having to go through your web site!) - these documents will be, in essence publicly available for download with no meaningful protection against that.
If you want to restrict access to downloads, you will need an extension and I would suggest that you search the Joomla! Extension Directory (JED).
On the JED, you could search using the following tag 'Paid Downloads' and get a list of both 'free' and 'paid' download extensions, along with details and reviews for each extension.