PDF link works, but file not showing

Hi – I am quite a noob with WP so apologies if this is a dumb question.

I’ve successfully followed all the steps to capture someone’s email prior to them receiving the download link to my PDF.

The link pops up and when you click on it, a download.php window opens.
The PDF loads to about 98% and hangs, and the PDF doesn’t display. However in the PDF viewer window, if I click on the “download to file” and then click Save As, I can download the file and successfully open it.
The file is a little over 1MB in size.

This seems to be a browser issue. I am using FireFox as my primary browser but I’ve tested it in Chrome, IE and Safari on my iPad and don’t seem to have the same issue – ie I can see the PDF (and download it too).

Any hints that might solve the issue for FireFox?

BTW – the following tutorial by WHAMDOM was awesome help:
https://www.youtube.com/watch?v=9Bb5Lo-KBPQ

https://wordpress.org/plugins/email-before-download/