While on the face of it this seems easy, the export options are difficult to get hold of. You can get the toolstrip of the reportviewer by simply doing this:
Dim myToolStrip As ToolStrip = DirectCast(ReportViewer1.Controls.Find("toolStrip1", True)(0), ToolStrip)
...and you can iterate through the .Items collection to do what you like with the buttons, however the DropDownItems collection for the export button always appears empty.
So the easy solution is to get rid of the default export button and add your own with just the functionality you need. So in your form constructor:
//Hide the default export button
ReportViewer1.ShowExportButton = False
//Define a new button
Dim newExportButton As New ToolStripButton("Export PDF", Nothing, AddressOf Me.ExportPDF, "newExport")
//And add it to the toolstrip
DirectCast(ReportViewer1.Controls.Find("toolStrip1", True)(0), ToolStrip).Items.Add(newExportButton)
Then all you need to do is to take care of the actual exporting:
Private Sub ExportPDF()
Dim warnings As Microsoft.Reporting.WinForms.Warning()
Dim streamids As String()
Dim mimeType As String = ""
Dim encoding As String = ""
Dim extension As String = ""
Dim bytes As Byte() = ReportViewer1.LocalReport.Render("PDF", Nothing, mimeType, encoding, extension, streamids, warnings)
Dim fs As New IO.FileStream("C:\export.pdf", IO.FileMode.Create)
fs.Write(bytes, 0, bytes.Length)
fs.Close()
fs.Dispose()
End Sub