Using MemoryStream Class to retrieve binary data for pdf and doc and allow download help

Hi,
 
Can anyone help on the above.  I'm not familar with MemoryStream Class however read the msn article at: http://msdn.microsoft.com/en-us/library/system.io.memorystream.aspx
But still not 100% sure on how to do the above.  Webservice will be sending across a pdf file as binary data and I need to store it in memory and then allow the user the ability to download it, code I have done to date below:
 
aspx:
<asp:ImageButton runat="Server" ID="imgDoc" CommandName='imgDoc' CommandArgument='<%# Eval("systemId") %>' ImageUrl="~/img/word-icon.gif" width="60" OnCommand="imgDoc_Click" /> <asp:ImageButton runat="Server" ID="imgPDF" CommandName='imgPDF' CommandArgument='<%# Eval("systemId") %>' width="60" ImageUrl="~/img/pdf-icon.gif" OnCommand="imgPDF_Click" /& ...