Not the parent but since I get most of my academic literature for university from libgen I usually have .pdfs. Also, I export my university transcripts to .pdf so there's that, too. I read them using a customized Zathura but I really like the concept you put up. I can imagine using this tool for learning for exams, as an example. Have you thought about making a local, offline client? Looks very promising!

Not the parent either but some texts and/or books are only available in PDF. I would never choose PDF when epub (or rtf, docx, txt) are available, but it happens that it is the only option.

Of course sometimes PDFs are just image files attached together and in that case it's impossible to do paragraph reading or any good stuff, but even in that case being able to synchronize position across devices would be a plus.