I pronounce it Pee-tah. It's an acronym Pee, Eee, Tee, Ae. If someone asked me "what's the first letter of the word ethical" I'd say E, not eh. If that makes sense.

Acronyms are often pronounced as a word unto themselves, not pronouncing each letter as it is pronounced in the word it stands for.

For example, in medical transcription CABG is called "cabbage" when it stands for coronary artery bypass graft. The A in the acronym is pronounced A as in apple, not Ah.

Another example: SCUBA. (self-contained underwater breathing apparatus) the U stands for "underwater" but you don't say "Scubb-a"