PDF on the Cloud
There are two primary models for working with PDF files on the cloud with Apple devices. One is to store PDF files in iCloud Drive and use apps like Preview to access the PDF. Another is to store PDF files in Apple Books and read it directly. Supposedly, the former is more flexible, while the latter is more focused. However, due to implementation issues, both models work awkwardly. First the iCloud Drive model, it's really flexible to use iCloud Drive to store whatever PDF at wherever organization finds suitable. With Preview on the Mac, reading PDF files is really tidy. For example, consider bunch of research papers and books in various folders on iCloud Drive. Classification can be added with Finder. Annotation can be added with Preview. The problem is that Apple didn't ship Preview on the iPad. Viewing PDF files in the Files app on the iPad just sucks. Then the Apple Books model, due to limited organization functionalities, it's best used to store only important PDF file...