Yeah, it's no longer the case that Whispersync only works on things bought from Amazon.
It would be less confusing if Amazon didn't call books from other sources "personal documents."
Here's a link:
http://www.amazon.com/gp/help/custom...deId=200767340
IIRC, all you have to do is e-mail the document to your Kindle and it's done automatically. But I may be wrong.