This add on is, by all appearances, "ancient" and the version noted on the page mentioned is 1.0. It may work, or it may not. The author's name is noted as well, so you could attempt to find them and make contact directly.
These days, if you have a recent (2013 or later) version of Microsoft Office, and you're looking to get plain text from a PDF, you can simply open it in MS-Word then do a save as in plain text. You will, of course, lose any graphics and alt-text associated with same and a great deal of formatting.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Puritanism: The haunting fear that someone, somewhere, may be happy.
~ H.L. Mencken