Thank you for sending the project.
We examined the produced PDF document and noticed that there is 6 fonts used for it. You could check which exactly they are when you open the document in Adobe - the Fonts tab from the Document Properties dialog will give you the information.
As these fonts are embedded in the document, they are increasing its size. In this case, even if you add more content in the document (without changing the styles and adding more fonts), you will notice that the size of the produced file won't increase much. This is because the biggest memory retainer are the embedded in the document fonts.
With this said, I am afraid I cannot suggest you an approach to decrease the size of the file.
Exporting RadRichTextBox to an image is not currently supported. However, we have this feature already in our backlog and you could vote and subscribe to the related public item
in order to receive updates if its status changes.
You could achieve similar functionality following the approach in the attached project. It demonstrates how to generate an image from the first page of the RadDocument using the RenderTargetBitmap class.
Hope this helps.
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