Invested the last two hours trying to fix a problem with Member PDF downloads on our website. Hopefully this solution will help someone else.
Our process for Members to login and access downloadable files via S2Member Pro has been basically the same for six years and has worked flawlessly. After shifting our development business a bit I no longer needed a dedicated server (for about $1,600 per year!) so moved my remaining sites to SiteGround's GoGeek plan. It was more than enough for our needs.
Trying to optimize all of my sites saw me (carefully) adding various methodologies and I hit a level of performance that was pretty close to my previous, dedicated server.
But then, this week, two customers contacted me that the PDFs they had downloaded were corrupt. I tried it myself and they were corrupt.
I began poring through everything I could find on file corruption, removed all the visible .htaccess additions, deleted server and browser caches, walked-back through all the changes I'd made at the server/.htaccess and Wordpress levels. I even tried logging in and downloading via Chrome, Safari, Firefox, and Opera, thinking it might be a browser issue.
Tech support nuked all caches at the server level as well, but the corrupt file problem remained.
On a whim I logged back in to the host and poked around CPanel. Turns out that I'd turned on Google PageSpeed which runs its own compressions which a tech support, server-level cache expulsion doesn't remove. I disabled it and deleted the cache.
Now all file downloads perform flawlessly again in all browsers!
Hope this helps someone else.