I have been using acrobat on the ipad for about 2 years and this issue only started about 3-4 months ago. When editing a pdf, normally, I write with the pen and double-tap to switch to eraser. This appears to only work intermittently or on certian pdfs and not others. Right now, I am consistently getting it to work on 3 specific pdfs (2 that I created via LaTeX, and 1 scanned in from paper by a student) but 2 pdfs it just won't work on (1 is LaTeX'd by me that it used to work on a few months ago, and 1 is LaTeX'd by someone else).