I have made a PDF from the HTML help. It is available
here.
The PDF is workmanlike, but it works well enough for me.
Please note: the PDF is offered as is, with no warranty of fitness for purpose (or any other kind of warranty). I have made the PDF available with the kind permission of The Omni Group, and any problems with the PDF are my fault and not theirs.
Hope you find it useful. I may update the PDF if a new version of OmniGraffle for iPad is released.
Jon