The manual is inside the app.
Press ⌘/ in Tandem Gander, or choose Help › Quick Start, to open the Quick Start. It is a PDF, read in the reader itself, and it walks through every control on the controller, the trackpad and the keyboard.
macOS 14 or later on an Apple silicon Mac. My development machine is a MacBook Pro with an M1 Pro. A DualSense controller is recommended, but the trackpad and keyboard do everything the controller does.
The shelf is empty at first. Choose the folder that holds your PDFs (⇧⌘O). The shelf shows the PDFs in that folder and its subfolders, and follows the folder as you add or remove files. I have deliberately left out file management beyond the classic drag and drop of a PDF into the app.
Some textbook PDFs carry data plots made of a great many line segments, and such a page takes seconds to draw the first time it is opened. The app shows a soft preview at once and the sharp page as soon as it is ready.