IFrame is stealing focus from main page controls

[DBxGlock]

Howdy folks,

I have an asp.net webpage that has a table with 2 columns. In the left
column is an IFrame. In the right column is a bunch of user controls
including a textbox, some buttons, a grid, etc.

Here's the problem: when the user clicks a button, the url of the
IFrame is set in the codebehind. When the page loads, the body's
onload event fires a javascript function that sets focus to the
textbox. A few seconds later, the IFrame finishes loading and the
focus is stolen away from the textbox and given to the IFrame.

Does anybody know how to stop this from happening? The IFrame's URL is
a PDF so I can't execute any JavaScript when it loads.

Thanks,
Dan