Adobe Acrobat DC is the default application and always has been so.
Looking at the Recent list again now, it's even weirder - it almost seems random. I'm seeing recently exported PDFs (perhaps all of them - I'm not sure), recently placed image files (some, not all), and a couple INDD files (some, not all). Here is an annotated screenshot of my current list:
As I said, I also have InDesign CC2017 Japanese, which I installed in February this year when I discovered that the English version didn't create the right kind of printer marks for Japanese printshops. I just now looked more closely at its recent list and did some testing (shown below). Although it has only INDD files (I exported a PDF just now, and it did not appear), it also has other mysteries. Only four of the files were actually opened in that version - the top two were today's test (a Save As and an Open) and the next two were in February. The rest were only opened in the English version! Most are older (from before I upgraded the English version from 2017 to 2018), so it's logical to assume that it adopted a list that was present on the system. But notice that two of the icons are different from the rest (I pointed at them with yellow arrows) - those are CC2018 files, and I think the rest are CC2017 (from before I upgraded the English installation). And neither of those icon types look like the white-bordered icon showing for all files in the above list. The fact that the icons in the above list are all the same for five totally different types of files is also an indication that something isn't working right.
Do these clues help you figure out what might be going on?