PDF Viewer for iOS supports Apple Pencil Double-Tap action out of the box. This behavior is activated as soon as an Apple Pencil is used to annotate a document.
Below is a list of the Double-Tap action configurations available in the main Settings app on iOS and how they translate in PDF Viewer.
1. Switch Between Current Tool and Eraser: If the currently selected tool supports erasing (ink tool and freeform highlight tool only), then it is switched to eraser. If the current tool is eraser and the previously selected tool supports erasing, then it will switch to the previously selected tool. In other cases, performing the double-tap action would do nothing in the app.
2. Switch Between Current Tool and Last Used: PDF Viewer switches to the previously selected tool. This switching of tools works for all annotation tools except for image annotation and stamp annotation i.e if the previously selected tool happened to be an image or a stamp annotation then the currently selected tool will get de-selected. Also, if the currently selected tool was selected twice in a row, then again the current tool gets de-selected.
3. Show Color Palette: The PDF Viewer apps shows the Annotation Style Inspector for the currently selected tool if the currently selected tool supports styling when this option is selected in the main Settings app on iOS.
4. Off: PDF Viewer does not perform any actions when this option is selected.