To view two documents or two different parts of the same PDF side by side within the PDF Reader app window, you can use the Split View mode.
1. Open a PDF and click the "Display" button in the bottom menu bar.
2. In the "Split View" section, you can choose to split the window vertically or horizontally.
3. In Split View mode, click "Select a File" to import a PDF from Mac Finder, or you can drag and drop a file to the secondary viewer window.
(Note: PDFs displayed in the secondary viewer are view-only and cannot be edited.)
4. Click "Replace" to change the current PDF in the secondary viewer.
5. To disable Split View mode, please right-click on the secondary viewer. Then choose "Disable Split View" from the pop-up menu.
6. Or click View from the top menu > Split View > Disable Split View