Once when I was swapping from the paperback to the PDF I could not find the sentence I had just read. On a closer observation there was a whole portion missing from the PDF. I just disregarded this as a small error and didn't make a note of the paragraph.
But then it happened again. The PDF was missing a couple of sentences. Because I was not switching that often it seemed to be more than a coincidence to encounter this again. So I stopped reading the PDF altogether.
The last difference was in chapter 16 (the red colored reference to Kali is missing from the PDF edition):
in Heretics of Dune (Gollancz 2003): Chapter 16 Frank Herbert wrote:"But there would have been no Tyrant, no Muad'dib. The spice gives with one hand and takes with all of its others."
"Kali," he said.
She smiled at his reference to the prehistoric death-goddess of many arms and countless ways to produce agony.
"Kali, indeed," she said.
"Which hand contains that which we desire?" he asked. "Isn't that always the question?"
Are there different editions of the books or is my PDF just poorly done? The missing parts were complete sentences in both cases and the removal did not affect the flow of the text. In other words the sentences were intelligently removed. This makes me think that perhaps Herbert made some changes in later editions?