We could do it by creating a stub class (PDFPage and/or PDFCircleElement in Alexander's example) with a special "new" that would first load the required package ?

next time you will be buying stuff in a supermarket you will think about me because you will not buy all the goods
in all the rows, Just the one you think you will eat and this is the same with Moose.
Why we cannot load XML parser when needed.
Because JSON, SVG, XML.... and output depends on the scenario.

I perfectly agree. Ideally, the image should be small, and support loading on demand. However, as fas as I know, there is no such infrastructure in Pharo. How can I load code only when it is needed?

I cannot realistically rewrite expressive like 

PDFPage new add: (PDFCircleElement new)

into:

((Smalltalk globals includesKey: #PDFPage) 

ifTrue: [ Smalltalk globals at: #PDFPage ]

ifFalse: [ “Gofer invocation to load PDF Exporter” 

Smalltalk globals at: #PDFPage ]) new add: (Smalltalk at: #PDFCircleElement) new

That would be insane. Loading on demand is indeed important. Java does it pretty well, Pharo should do it too.

But infrastructure (even a basic one) is needed for this.