For PDFs, I've been using Readdle PDF Expert or Readdle Documents for years now. It syncs with Dropbox (both ways), and I have huge collections of PDFs that I m

For PDFs, I've been using Readdle PDF Expert or Readdle Documents for years now. It syncs with Dropbox (both ways), and I have huge collections of PDFs that I mostly read on the iPad. A subset is also available on my iPhone. Highly recommended.