Hopefully my discoveries help someone to think this through...
I originally setup my iOS email as "Other" then added all the imap & smtp details etc per the guides. This account has the PDF problem.
As a test, I deleted the account from my device and re-added it but this time as an "Outlook.com" account. When I did that, the email synched and I could then open PDFs immediately. However, there is nowhere to set imap, smtp etc. It also doesn't allow me to link it to my alias email account which I also want...
Anyone have any comments on the above and why the difference in results for both setup types?
Trying to get my head around why Bigpond IMAP account doesn't like the PDF format...
Thanks