Click handlers for two to ten buttons
Naming:

Like the previous answer stated, your classnames are far from following conventions. The should be PascalCase and not camelCase. Also a name like dxRpt12MonthsItemPurchaseAnalysisByAmount is definitly not meaningful, not for you, not for others who read your code. Try names like FirstReportForm, FirstFilterForm or FilterForm1, ReportForm1 and so on.

Tip: Names of Classes, Structs, and Interfaces

Braces:

Microsoft doesn't have any conventions on the formatting of your braces. This means you can chose between following:

if (isValid) {
    DoSomething();
}


and 

if (isValid)
{
    DoSomething();
}


But it's best practice to stick with one and not to mix the two up in your code. (I prefer the second use of braces.)

Logical use of variables:

In both click event handlers you instantiate a new filter. Whether or not document is null, you call CreateWindows with again a new instance of that filter-class. Just re-use the instance you created before. You didn't manipulate it in any way.

Generic method:

First of all, the following code is written out of my head and not tested as I have little information what all your classes are/mean. But here's an attempt to make it generic:

private void HandleButtonClick&lt;T&gt;(Form filterForm) where T : Form, new()
{
    var document = FindDocument&lt;T&gt;();

    if(document == null)
    {
        CreateWindows(filterForm, new T(), sender, e);
    }
    else
    {
        tabbedView1.Controller.Activate(document);
        if (this.ActiveMdiChild is T)
        {
            filterForm.Focus();
        }
    }
}


And the usage of the method:

private void button1_ItemClick(object sender, ItemClickEventArgs e)
{
    HandleButtonClick&lt;ReportForm1&gt;(new FilterForm1());
}

private void button2_ItemClick(object sender, ItemClickEventArgs e)
{
    HandleButtonClick&lt;ReportForm2&gt;(new FilterForm2());
}


Notes:


T is the type of the report form
The new() constraint is set on the method, check tip for more info
I reversed the null check for readability, this might be personal choice


Tip: new Constraint (C# Reference)

I cannot reproduce your code and therefore cannot test the method. Please let me know if you got it to work using this method. Hope this helps!