This property determines the resolution of the output image.
Because PDF documents specify distances in physical units
(typically points), a resolution unit is needed to convert these
physical units to pixel based units.
For example, a document 612 points wide rendered at 144 DPI will
result in an output image 1224 pixels wide.
Changing this property will change the values of both the
DotsPerInchX and DotsPerInchY properties.