Background: I purchased Papers 2, a reference management program for the Mac, from its publisher (Mekentosj Software) way back in 2012. At the same time I also installed the Papers iOS app on my (then-new) iPad Mini. I really appreciated the simplicity with which I could sync my entire library of research papers (references, PDFs and all) between the two devices -- it really lived up to its nickname of "iTunes for PDFs", which was all I wanted.
Shortly after I got all set up, Mekentosj introduced an updated version of their software, Papers 3, but based on what I read online from dissatisfied users I decided to pass on the update. I continue to use my older versions to this day and am still very happy with them.
However, I recently purchased a newer iPad (7th generation) that runs iOS 14.1, and the original Papers app for iOS will not work on the newer device. It turns out there is an updated "Papers 3" app for iOS, but it can't sync with Papers 2. And the Mac version of Papers 3 is not even available for purchase anymore -- Mekentosj has since sold the product to Readcube, which has rolled out yet another version ("Papers App by Readcube") which I am uninterested in, not least because it requires a monthly subscription rather than a one-time purchase.
All I want to do is sync my Papers 2 library to something that will let me read PDFs on my new iPad. (I can still do this on my old iPad Mini just fine, but that is no longer my primary device.) Does anybody have any recommendations for how to best do this? If I have to start all over on a new platform (e.g. Mendeley or Zotero, neither of which I have ever used) what is the easiest way to migrate everything over and get up and running?