It rotates PDF content according to selected paper orientation.
If content of the page has greater width than height, and paper orientation is Portrait than it will rotate the content for 270 degrees.
If content of the page has greater height than width, and paper orientation is Portrait than content will stay the same.
If content of the page has greater height than width, and paper orientation is Landscape than it will rotate the content for 270 degrees.
If content of the page has greater height than width, and paper orientation is Landscape than content will stay the same.
For default Print() method AutoRotate works per PDF page. If PDF document has more pages with different pages for every one of them will be applied described logic.
For PrintWithAdobe() method orientation is determined based on the content of the first page which will be printed. That can create problem if pages with different page sizes are printed. That is one of the know limitations of PrintWithAdobe() method.