X-Frame-Options: ALLOW FROM URI Will Not Display iFrame

Question

I have been trying to allow another website to display an iFRAME of a pdf from our website. When they try to iFRAME it is blank and the error is:

Load denied by X-Frame-Options: http://longurl.pdf does not permit cross-origin framing.

I attribute this error to our site as inspecting the headers shows me:

X-Frame-Options: SAMEORIGIN

It is a wordrpess/Genesis site so I added in my functions.php

add_action('send_headers', 'xframe_options');
function xframe_options(){
    header("X-Frame-Options: ALLOW-FROM https://amdsb-on.safeschools.com/");
}

I am still getting the blank page and the exact same error on the https://amdsb-on.safeschools.com/ site when trying to display the iFRAME

When I inspect my site now I see:

X-Frame-Options: ALLOW-FROM https://amdsb-on.safeschools.com/, SAMEORIGIN

All my research tells me this is what you should do to allow a specific site to iFRAME yours. is it because it is a pdf? Is it because one site has SSL and one doesn't?

Thanks for any help you can provide.