I would be hesitant to comment specifically about Ontario PNP.
But CIC officially provides ways around it. Your proof of income tax, last years filing etc. combined with copies of any kind of official correspondence that you can provide is acceptable.
You might wanna go through some old threads for details too.
http://www.canadavisa.com/canada-immigration-discussion-board/what-to-do-when-employer-refuses-to-provide-letter-of-employment-t137182.0.html