So, it turns out it was SnapPDF that was failing to load a bunch of dependencies which caused the “Failed to load PDF” error message.
This was found by trying to create the ‘Hello World’ test pdf that’s been mentioned elsewhere on this forum. Error message indicated that I had dependencies that were missing.
This link showed me all of the dependencies for my installation — installed them all, and now my PDFs are able to be opened (even newly created ones).