How can we shape a document after mailmerge.
After mailmerging a document, text appear automatically in the document.
But we want the document to be well formed, for example paragraph will starts in a new page.
Sorry, not exactly sure what do you mean.
If you have line breaks in your data, it should be inserted into the document with paragraph breaks.
You can use DocumentBuilder inside merge field event handler to insert or format complex text during mail merge, see the demo project, Alternating Rows report for an example.