How do I view page headers when I receive Error 400?

I have a php web page with a fillable pdf embedded in an <object> element. I have searched for hours for a way to submit the filled-in pdf to a script for processing or emailing. This question has been asked multiple times on SO with no answer. So I am trying to create that solution by starting with the basics.

Adobe Acrobat includes a function that can submit the filled-in pdf by invoking mailto: which fails to work on two out of three computers. Also, fails on all computers that do not have an email client installed.

So I am now using a Submit button built into the pdf that uses the Adobe built-in function submitForm to invoke a php page that can read whatever it is that the function submits. I cannot find any documentation on what to expect from the submitForm function, so I thought I would have the invoked php page simply print out all the global variables. But the page won't even load. I get: Bad Request - Invalid Header HTTP Error 400. The request has an invalid header name.

So, what I am trying to figure out is, how do I see the actual headers that the server says are invalid. I operate the Windows server and have full access. I looked through the Events logs and every IIS log I could find. I can't find anyway to view what headers are being sent that are invalid.

Again, how can I see the headers that are being sent?