A number of mobile browsers don't save the file correctly when the Generate button is pressed. Based on my experience just now, one popular browser saves a copy of the entire web page to the .ovpn, another just saves blank files.
Rather than try to debug all of these I have a suggestion. Offer a checkbox or button on the Generator page that will display the config right there, in a box, such as an edit field, allowing the user to simply select and copy it to their paste buffer. This will get around filesystem quirks and give the user some flexibility in how to save it on their device.
What's more if it's an edit field (textarea) and the user knows of changes they need to make or would like to insert a comment, they'd have the handy ability to do it right there in the web page before copying it.
Thanks for your consideration.