How official is the word "subpar"?
Is there a reason not to use it in a document and are there any circumstances under which I should avoid its use? Could the phase "subpar performance" confuse the reader?
This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information. See the FAQ for guidance on how to improve it.
There is no official academy of the English language. Words are neither official nor unofficial.