PDF is a file format - the files on the CD are in pdf format - the CD is physically a thin plastic disk which is breakable (I have had a report of one american postie folding an envelope containing one to get into a post box) - however the usual clam shell CD case is even more fragile and weighs much more than the CD - my solution (and what do I know as have only posted a couple of hundred without any breakages except the above) is to use a card stiffener in an envelope with the CD in a plastic or paper sleeve - the envelopes are non standard size (but often used for Xmas cards) however a local firm will make envelopes of any size if asked - the combined weight allows a CD to be sent at letter rate for airmail.

I still have had no response to my email to the society - I suspect the mailbox is not being checked but this is par for course.

The MI books could also be done in this format - there is enough room on a CD to contain all Burial + MIs on a single Cd as each of the digitised burial book in pdf format occupy about 2MB and you get over 600MB to a CD.

I wouldn't bother with the the 1851 census as this contains too many errors and there are better versions available