Short answer:

"Pd" and "Pure Data".

Longer answer:

On the mailing list or on your personal pages, feel free to use whatever you like best. Most of the time people will understand what you mean. That is, don't use something like "pD", which is not common at all.

But if you are writing about Pd in some kind of offical document, e.g. in a conference paper for ICMC, you may want to use the name, Pd's inventor, Miller Puckette uses consistently in all his publications on the topic: "Pd" and "Pure Data"

These publications include the very first appearance of Pd in: Puckette, M. 1996. Pure Data: another integrated computer music environment., all the other papers on Miller's site as well as his book Theory and Techniques of Electronic Music