LiquidText is the kind of app that I wish I had access to back when I was in school.
You can get PDF Expert on the App Store for $9.99, while another $9.99 for the in-app purchase will unlock the professional-level PDF editor, which lets you completely edit the content of PDFs like text, images, and links. In addition to local files, you can also link cloud services like Google Drive, Microsoft OneDrive, Box, and Dropbox to PDF Expert, and everything can be kept in sync between your devices with iCloud.
There's even a built-in media player, which allows you to play audio and video files that you have stored. But while it's primary focus is that type of file, you can use PDF Expert to manage and view several file types, including Google documents and photos. While I mostly use Files these days as a central location for all of my.files, PDF Expert is my go-to for working with PDFs in almost any capacity. PDF Expert is a long-serving app in my personal rotation for a very good reason: it's great at what it does, and what it does is quite a bit.