1. Not available online doesn't mean the paper's existence is made up. It's a very bold claim to make for the authors that they cite work that is fabricated.
From the available information, this looks like a technical report by a, probably now defunct, company back in the 80s. If this was its only form of publication, and not on some conference proceedings for example, it would be only found available on select university libraries as a physical copy. But most important,
2. This isn't even as an impactful paper as the parent comment states. Or if its proposed concept is, the original idea is probably derived from some other paper that is indeed the one that is highly cited and most definitely available online.
Accumulative citations number from Google Scholar and IEEEXplore doesn't exceed fifteen for the particular paper though.
https://scholar.google.com/scholar?cites=1491448744595502026...