This code demonstrates how to save WinForms controls in Word the official documentation for Microsoft does not give the code for how to save winform controls when the word document is saved as said by Microsoft "Windows Forms controls are not persisted when the document is saved and then closed"

firstly create a button on the ribbon to generate a button on runtime. and dont forget to add refernce to Microsoft.Office.Tools.Word.v4.0.Utilities.dl then on the button click event we added this piece of code to reference a method in ThisAddsIn.cs Class

private void btnTest_Click(object sender, RibbonControlEventArgs e)
{
  Globals.ThisAddIn.WhenRibionBtnIsClicked();
}

now we go to the ThisAddsIn.cs Class and Make a method called WhenRibionBtnIsClicked()

    internal  void  WhenRibionBtnIsClicked()
    {
    Document vstoDocument = Globals.Factory.GetVstoObject(this.Application.ActiveDocument);
     Word.Selection selection = this.Application.Selection;
      if (selection != null && selection.Range != null)
	    {
	    //Generate button on run time
	    	string name = "myBtn";
	    	Button button = new Button();
	    	button.Click += new EventHandler(Generatedbtn_Click);
	    	button = vstoDocument.Controls.AddButton(selection.Range, 100, 30, name);
	    	button.Click += Generatedbtn_Click; //for the click function
	    	button.Text = "I am A Generated Button";
	    	button.Name = name;
	        }

next we need to save the button state to a location you can do this multiple ways the best way to do it is store it in the documents custom properties what i did was create a function to achieve this

  void saveInfoInProp(string name,string value, Microsoft.Office.Core.DocumentProperties properties)
    {
        //save the start range pos
        if (properties.Cast<DocumentProperty>().Where(c => c.Name == "startPosition").Count() == 0)
        {
            properties.Add(name, false, MsoDocProperties.msoPropertyTypeString, value);
        }
        else
        {
            properties[name].Value = value;
            Globals.ThisAddIn.Application.ActiveDocument.Saved = false; //important somtimes
        }
    }

the way we used this was

string startPosition = selection.Range.Start.ToString();//get start position
string endPosition = selection.Range.End.ToString();//get endposition
    //save the start range pos
    saveInfoInProp("startPosition", startPosition, properties);
this function saves the start position in to the properties of the document so we can later make use of them when the document starts up

so now we are done with creating the button, next we need to recreate this button once the Word document starts with so this needs to be coded in the ThisAddIn_Startup function

private void ThisAddIn_Startup(object sender, System.EventArgs e)
{
    Word.Application wb;
    wb = this.Application;
    if (wb.Documents.Count > 0)
    {
    	String queryResult_StartPosition = String.Empty;
    	String queryResult_EndPosition = String.Empty;
    	String queryResult_btnName = String.Empty;
    	String queryResult_btnText = String.Empty;
    	Microsoft.Office.Core.DocumentProperties properties = (DocumentProperties)Globals.ThisAddIn.Application.ActiveDocument.CustomDocumentProperties;
    	//get doc info about start pos
    	queryResult_StartPosition = loadInfoInProp("startPosition", properties);
    	//get doc info about end pos
    	queryResult_EndPosition = loadInfoInProp("endPosition", properties);
    	//get info about btn name
    	queryResult_btnName = loadInfoInProp("btnName", properties);
    	//get info about btn text
    	queryResult_btnText = loadInfoInProp("btnText", properties);
    	Document vstoDocument = Globals.Factory.GetVstoObject(this.Application.ActiveDocument);
    
    	Word.Range rng = vstoDocument.Range(queryResult_StartPosition, queryResult_EndPosition);
    	Word.Selection selection = this.Application.Selection;
    	if (selection != null && rng != null)
    	{
    	Button button = new Button();
    	button.Click += new EventHandler(Generatedbtn_Click);
    	button = vstoDocument.Controls.AddButton(rng, 100, 30, queryResult_btnName);
    	button.Click += Generatedbtn_Click;
    	button.Text = queryResult_btnText;
    	button.Name = queryResult_btnName;
    	}
    	}//end of wb.docment.count
    }

there is of course a function to load what ever was stored in the property

string loadInfoInProp(string name, Microsoft.Office.Core.DocumentProperties properties)

{
	string value;
	if (properties.Cast<DocumentProperty>().Where(c => c.Name == "startPosition").Count() > 0)
	{
		value = properties[name].Value;
	}
	else
	{
		value = String.Empty;
	}
	return value;
}

and now to code a click event handler for this is easy since you attached a event handle

void Generatedbtn_Click(object sender, EventArgs e)
{
 MessageBox.Show("I have Been Clicked :-O");
}