Sounds like a plan. And you should never ever change core files. Try making overrides for views or try using Rereplacer from Regularlabs first to make your changes. 
Or keep a log file somewhere with your changes if everything else fails.
And maybe disable JCH optimize to test. Might be the cause of your problem as well.

I've looked into this. The PDF part of Kunena hasn't changed between K6.4 and K7.0: pdf.php and the getUrl() method in KunenaAttachment work the same way in both versions. So this isn't caused by Kunena 7 itself.
I checked the direct URL of one of your PDFs (/media/kunena/attachments/773/20260709_075242.pdf): it returns 200 OK with Content-Type: application/pdf, so the .htaccess doesn't seem to block the file itself.

A few things I noticed on your site:

1. Your attachments aren't protected (the PDF has its normal filename), so that setting isn't the issue.
2. The PDF is served correctly by your server (Content-Type: application/pdf, same origin, www redirect is fine).
3. The wf-responsive classes and the ?ts= parameter on your <object> come from the Responsify plugin (by the JCE developers), not from Kunena. I tested Responsify on my own test site, though, and the PDF still displayed fine there.

Could you please:

1. Restore the original pdf.php (remove your substr change)
2. Clear the JCH Optimize cache and do a hard refresh (Ctrl+F5)
3. If the PDF still doesn't show, open the browser console (F12) and post any error messages here, together with the rendered <object> line from the page source.

That way we can see what is actually blocking it on your site.