I can't draw on my PDF! The scribble tool isn't working.
If drawing with your finger while in scribble mode isn't working, it could be you have set up a Bluetooth stylus or Apple Pencil. While the stylus is selected in the Settings, your finger or the Apple Pencil will not work in its stead.
Open Tools > Settings > Stylus and tap "None" to deselect a stylus. Thereafter, your finger or the Apple Pencil will work.
(Applies to PDFpen 6 and later).