There are several forms of notarized documents, the most common of which are affidavits (which are written statements of a person made and signed under oath) and acknowledgements (which is a notarized statement that a document was signed before a notary, usually used for documents related to real property).
There would be no legitimate reason I can think of to notarize an offer letter, and the fact that you want to do that probably means that you are confused about some other aspect of the law that would make you think that you would want to or need to do something like that.
A more common thing to do, for example, in anticipation of a lawsuit, would be to prepare and execute an affidavit which states that the letter, attached as an exhibit, was signed by you, and anyone else that you have personal knowledge signed it, for your signature before a notary.
A notary is not allowed to execute an affidavit (or a very similar document called a verification which is a very short document saying that the facts stated in a single document asserting a claim in a court case are true and correct in a form substantially similar to an affidavit) if it is signed outside the notary's physical presence (in theory because the notary administers an oath before you sign it).
An acknowledgement can be executed by a notary if the person who signed the document comes before the notary in person and acknowledges that it was signed by him or her, even if the notary was not there when it was actually signed. The notary would state the date that you acknowledged it in person to the notary in the acknowledgement and would not make any statement regarding when it was actually signed.
There is special formalized legal language that must accompany each kind of notarization, which is called the "jurat". Then the notary signs and dates the jurat in the appropriate place and applies a notary seal near the jurat in the indicated location (if any) mark "L.S." for "location of seal" in latin.