You could use a simple extension method:

    public static void PasteIntoCurrentTab(this TabControl tabControl)
    {
        if (tabControl.SelectedTab == null)
        {
            // Could throw here.
            return;
        }

        if (tabControl.SelectedTab.Controls.Count == 0)
        {
            // Could throw here.
            return;
        }

        RichTextBox textBox = tabControl.SelectedTab.Controls[0] as RichTextBox;
        if (textBox == null)
        {
            // Could throw here.
            return;
        }

        textBox.Paste();                    
    }

Usage:

myTabControl.PasteIntoCurrentTab();

I suggest you keep some "current state" variables updated so you always have a pointer to the selected Tab Page, and its child control (in the case of a tabbed-notepad emulation discussed here : a TextBox). My preference would be to keep track of the TabPage<>TextBox connections using a Dictionary to avoid having to cast the TextBoxes if they are accessed using the TabPage.Controls route : the following code assumes you have a TabControl named tabControl1 on a Form :

Dictionary<TabPage, TextBox> dct_TabPageToTextBox;

int tabCnt = 1;

TabPage currentTabPage;

TextBox currentTextBox;

So, as you create each new TabPage at run-time you call something like this :

    private void AddNewTabPage()
    {
        if (dct_TabPageToTextBox == null) dct_TabPageToTextBox = new Dictionary<TabPage, TextBox>();

        currentTabPage = new TabPage("Page " + tabCnt.ToString());
        tabControl1.TabPages.Add(currentTabPage);

        currentTextBox = new TextBox();

        dct_TabPageToTextBox.Add(currentTabPage, currentTextBox);

        currentTabPage.Controls.Add(currentTextBox);
        currentTextBox.Dock = DockStyle.Fill;

        currentTextBox.Text = "sample text for page " + tabCnt.ToString();

        tabControl1.SelectedTab = currentTabPage;
        tabCnt++;
    }

As the end-user changes the selected TabPage you can simply update your current state variables like this :

private void tabControl1_SelectedIndexChanged(object sender, EventArgs e)
{
    currentTabPage = tabControl1.SelectedTab;
    currentTextBox = dct_TabPageToTextBox[currentTabPage];

    MessageBox.Show("text in current Tab Page is : " + currentTextBox.Text);
}

So now have the code that is invoked by your menu choices applied only to the currentTextBox.

best, Bill

I tried this for fun ... I made a form with a ToolStripContainer, and a ToolStrip inside it, with the standard buttons (which includes the paste button). I renamed the paste button to pasteButton, and hooking everything up you get:

public Form2()
    {
        InitializeComponent();

        TabControl tc = new TabControl();
        toolStripContainer1.ContentPanel.Controls.Add(tc);
        tc.Dock = DockStyle.Fill;

        TextBox selectedTextBox = null;

        pasteButton.Click += (_, __) => selectedTextBox.Paste(Clipboard.GetText(TextDataFormat.Text));

        int pages = 0;

        newTabButton.Click += (_,__) => {                
            TextBox tb = new TextBox { Multiline = true, Dock = DockStyle.Fill, ScrollBars = ScrollBars.Vertical };
            TabPage tp = new TabPage("Page " + (++pages).ToString());                
            tc.Selected += (o, e) => selectedTextBox = e.TabPage == tp ? tb: selectedTextBox;
            tp.Controls.Add(tb);
            tc.TabPages.Add(tp);
            tc.SelectedTab = tp;
            selectedTextBox = tb;
        };           

    }