Second VikingOSX's suggestion. If I hadn't already purchased Master PDF Editor, I would have gotten a cheap license for Acrobat Pro there instead.
If you're wondering how they do this kind of pricing, these are considered sort of gray market sales. Microsoft and other vendors seems to be okay with them since they've never made any attempt to shut such sales down.
Where all of these cheap licenses come from are large corporations that routinely replace hundreds, or thousands of workstations every three years or so. They already have a site license for all of the software they use, so all of those new computers have valid product codes on them that will never be used. That is, an OS or other bundled software that was paid for as part of the cost of the computer, but never activated.
They give, or sell all of these otherwise wasted codes to places like Stack Social.