Personally, I like some margin and I prefer only left justification rather than full justification. So the only issue with a PDF file is that the reader doesn't get to chose which he/she prefers.
Since I have a Kindle, I had to send your PDF file to amazon to format it for me. I think it pulls the text out and makes it a mobipocket file. Which means, of course, that it's no longer a fixed format, but is now free flowable text and I can choose my own font size or justification. In other words, I can't really view your PDF as you created it! (Except on my mac, of course.)
As long as your PPD file creates text big enough to be seen on a 6" screen, it is probably fine. PDF's are still not *ideal* in that the user cannot have her own choice of how to display. But if it's the only format available (and if it is sized for a 6" screen) it is probably fine. IMHO