A layer of visible content on a page.
Each time content is added to a page a new layer is created.
Different kinds of layers are created for different types of
content. For example the AddText method will result in the creation
of a TextLayer and the AddImage one will produce an ImageLayer.
A layer corresponds to a stream object referenced from the Page
Contents array. The Page.Flatten method concatenates all the stream
and then compresses them to save space.
Note that this type of layer is an ABCpdf construct that you
cannot detect using Acrobat. Acrobat layers are something
completely different and are more precisely known as Optional
Content Groups (OCGs). See the Doc.Layer property
for details of how to construct and manipulate this type of