To save complete document to rtf:
Document doc = new Document("MyFile.doc");
To save only a fragment of document to rtf is a bit tricky at the moment. You need to either delete all nodes from the document except the ones you want to save and then save. Alternatively you can copy nodes you want to save into a blank document and then save it. Either way it requires dealing directly with nodes of the document. For example if you want to save just a single table to rtf:
Document srcDoc = new Document("MyFile.doc");
// Let's say the table we want to save is the first table in the document, get it.
Table srcTable = (Table)srcDoc.GetChild(NodeType.Table, 0, true);
Document dstDoc = new Document();
// This creates a copy of the table ready for adding to the new document.
Table dstTable = dstDoc.ImportNode(srcTable, true);
//Add the table to the main text of the first section.
// Now dstDoc contains only the table, export to RTF.