I'm trying to write a process that fills out IRS form 720 and then downloads it.  I've downloaded the form from the IRS website, added it as a resource and been able to get the fields list and assign a few of their values. 

But when I attempt to set the form field values, I get a Field is Read Only error.

I'm able to fill and save the pdf with Adobe, so why can't I set the data fields here?

Thanks.

I'm attaching the IRS form.