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The higher level building blocks of a PDF document. The PD level objects carry 
the PDF document semantics like "this is a page" and "this is a image".

The PD objects itself are built upon COS objects, sometimes primitives like 
COSString or COSName, but most often COSDictionary and COSStream.



This implementation is guided by the following rules:

META class structure:

Every PD class is accompanied with a META class defining META information and acting as a factory.
Factory creation:

The PD level object is created or looked up using the factory methods provided by its META class.
Identity:

A PD object is always 1:1 associated with a COSObject.
COSObject and Java return types:

The return types of the PDObject level methods are as follows: 
All "cos" prefixed methods reference back to the COS level and return COSObjects. 
All other methods return (or act upon) either higher level data structures like CDS or PD itself or plain Java objects.
COSObject, COSNull and plain null:

All methods with a COSObject in return signature will never return plain Java null, but COSNull.
Methods that are already narrowed to a specific subtype, like COSName, will return either
an instance of that type or Java null.