I found Zotero to be a nice way to organize large numbers of papers. But their mobile story was a bit lacking as of a few years ago.

Thank you. Exactly what I was looking for!

I use Calibre [0] for ebooks, and PDFs are just in a file on the desktop. I supose you could put them into calibre, but reading papers on an eink reader doesn't sound idea.

[0] - https://calibre-ebook.com/

Thank you, looks nice. Is it mainly for e-books, though?

Calibre for e-books.

+1 for Calibre

I use a "heap" data structure :-)