Thank you for contacting us!
Regarding your comment that you'd like to put a RadPdfViewer next to the RadRichTextBox in your project and integrate them, I suggest you take a look at this demo
(if you haven't seen it already) that shows the exact approach in which this can be achieved.
As for your questions - unfortunately, the margins you painted red in the attached picture cannot be easily removed. Regarding the small space on top and on the left of the ruler, you can try changing that space by modifying the template of the DocumentRuler. As for the space between the ruler and the page, unfortunately it is not possible to modify it.
To shed some light on this, the space is actually dynamically changed which you can verify by resizing the browser window in which you've placed RadRichTextBox. Generally, the only option in this regard would be to change the layout mode of the editor, for example to Flow and no margins will be visible.
As for your concerns about zooming - it is not possible to change the place to which the zoom in/out centers. I logged this for further revision and we might reconsider the current behavior in the future, so that zoom in/out will follow the caret position or the last viewport. However, the task is with low priority and I cannot tell if and when such change might be implemented.
I am sorry for any inconvenience these issues might be causing you! Let us know if you have other comments or questions.