Have you found solution? I meet the same trouble, I only know that the eraser's width changing following with touch.altitudeAngle values, so, I try to find some approach to set a fixed touch.altitudeAngle value when canvas's PKTool was PKEraserTool. But not success so far.
M-Three
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Correct some my descriptions (more detail)：
Correct: PKCanvasViewDelegate inherits UIScrollViewDelegate, So it can conform all UIScrollViewDelegate's methods.
However, I need to add PKCanvasView to another view(PDFView's documentView), so that can doodle on PDFDocument. I had already achieved it. But still when zooming PDFView, my PKCanvasView that as subview of PDFView's documentView was scaled too. Then the drawing was blurred. How to fix it?