Some formats such as SVG contain a viewport wrapper and then a
viewbox containing content.
The size of the viewport can be specified - much the same as
specifying the size of paper on which something should be
However similarly you can specify the size of the content
without specifying the size of the viewport - much the same as
saying that the drawing is a certain size without dictating the
size of the paper.
This is quite a common situation for web output where you want
the content to be drawn in a browser window and you do not know
what size the user will select for that window.
If this property is set to true, the page size will be set to be
the same as the content viewbox. This effectively shrinks the page
to fit the content avoiding excessive white space or clipping.
If this property is set to false, the page size is taken from
the DefaultRect property and the
content is centered on this page. The viewbox size is inserted as
the page ArtBox
for possible use at a later stage.