Specifies the media box defining the boundaries of the physical medium on which the page is to be printed.
The media box may include any extended area surrounding the finished page for bleed, printing marks, or other such purposes. It may also include areas close to the edges of the medium that cannot be marked because of physical limitations of the output device.
Discarding the content that falls outside the media box boundary will not affect the meaning of the PDF file.
The PDF page unit of measurement is points, which measures 1/72 of an inch (0.3528 mm). You can convert this unit of measurement, e.g., to pixels using the Units.PointsToPixelsF method.