How to preserve formatted bookmark text?

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C#3.5

Hi I am having a document which has several bookmarks. Each bookmark may contain some text either plain text or formatted text(HTML). Now The text for each bookmark is extracted from database. And I download the document. I made several changes such as changing font color, font size and underline the font etc.,. Then I extract each bookmark text to database and I update each column where from I got the previous value for the respective bookmark. After the updation, Again I load the document with updated formatted text but I only got the plain text instead of formatted text.

For Example I have 3 bookmarks in a document
Name: Vinoth whose bookmark.Text is <b>Vinoth</b>
City: Chennai whose bookmark.Text is <u>Chennai</u>
Country: India whose bookmark.Text is <font size='5'>India</font>
I used DocumentBuilder.MoveToBookmark(Bookmark.Name) and InsertHtml(Bookmark.Text) to get the document with formatted text and I download the document.

Now I am updating the formatted text in the downloaded document as follows
Name: Vinoth -font size has changed
City: Chennai-font fore color has changed and underline is removed
Counrty: India-font style has changed

After making changes in the formatted text I update all the bookmark texts to database using c# coding.I can save all bookmark's text as plain text to database. Again I am loading the document with updated formatted text. But I only got plain text in resultant document as follows
Name: Vinoth
City: Chennai
Country: India
Could you tell me the solution to get the updated formatted text in the bookmark's text using c#.

string text = doc.FirstSection.Body.ToString(SaveFormat.Html);

reads all the text into a single string variable. I need each bookmark text in individual string variable.