My wife received a letter from the Embassy of Pakistan with her interview date. It says
Copy of Naturalization certifate from American Citizen Petitioner. What document is this? Can anyone assist me? Thanks.

I assume you are not a natural born citizen of the US and gained citizenship through Naturalization. There is a certificate that you receive when you become a US citizen, and they are asking for a copy of this (as proof of your citizenship) for your wife's interview.

Don't worry that the certificate says you are not supposed to make copies of it. I found out that it is ok to make copies if the copies are for US govt. use.

Are you a naturalized US Citizen? Do you remember getting one certificate during the citizenship Oath Ceremony? That is the naturalization certificate.

Sorry guys after researching online its also known as a PASSPORT. I was born in the United States so I should just send a copy of my passport in this case right? Thanks

Right! You can send the passport copy as well.