I am having the exact same issue. Did you ever figure out a way to fix this?
You did not indicate how you are creating the PDF. My guess is that the actual problem is the e-mail aspect. You have to be sure the PDF file is encoded (not sent as ASCII) or zipped for transmission in e-mail. It may be the MAC user is also using the MAC preview and not an Adobe product. So as you might see, there are several potential problems and there is not enough information to hone it down. Maybe the comments help some.
I was converting a MS word 2010 doc to a PDF. I think I found a solution though. When saving the PDF, I selected the "options" and checked off the box under PDF options for "ISO 19005-1 compliant (PDF/A). I tried to access this updated file from my iPad and it no longer displayed the text as black lines. It did, however, increase the file size quite a bit... I think that is due to embedding the fonts? Anyhow, it appears to be working now :-)