I work in a support organization and we've seen variations of this issue develop on various iphones and ipads with ios7. Oddly, not everyone is experiencing this, just some people.
In my own case, I simply downloaded Acrobat Reader for free and can read the things fine.
We use an exchange activesync email server and when each email comes in it indicates at the very bottom that it has been downloaded as plain text and to click to download the entire message. I've tried clicking to download and not clicking and I can read the PDF either way.
In my own case I can view PDF files and/or also save them by pressing and holding the icon of the PDF in the email.
The other thing one of our end users noticed: if she emailed the PDF directly to her iPhone5 and iPad it would not be readable but if she forwarded it to her offsite Yahoo account and then forward to her corporate email it would open.
I think there's definetely something going on with the formatting of the PDF (was it created by a true Adobe PDF distiller or one of the many clones?), what email application attached it? (Outlook, Eudora, IOS Mail, etc) and how many PDF reading apps are on your IOS devices?
For instance I have the default PDF reader, iBooks, Kindle, Adobe Acrobat Reader, and there may be others that I don't even know has PDF capability. Only Apple probably has a good idea of how many apps utilize this.
Message was edited by: aliensporebomb - clarification