Response.AppendHeader & Content-Disposition

Background
btnPreview & btnExport both generate a report document that is
either viewed in the browser or made available to download as a
file.  The document generation process can take more than 10
seconds or so which is typically longer than your average user will
wait before hitting btnPreview or btnExport again.  What I've done
is to disable these buttons using JavaScript when one is clicked. 
The buttons are then enabled after the routine is finished.  


This works perfectly for btnPreview.  btnExport works with the
exception of enabling the buttons when finished.  I believe I've
narrowed the cause down to the following line in the btnExport.Click
event:

    Response.AppendHeader("Content-Disposition", "attachment; filename=report.pdf")

I've tried adding code in the button.click events to explicitly enable the buttons but they stay disabled. 

Any ideas woul ...