Large PPT Files Break On Download

I set up a webpage that allows users with certain privileges to download protected slideshows (.pptx files) for a fee. For some reason any files bigger than 100MB break when the user on a Mac computer opens them after downloading. There are two sets of files, one tailored towards PC users and one towards Mac users. The users on PCs don't experience any issues when they download the PC versions of them. I'm asking about it here because as far as I can tell it's an issue with the machines downloading and not the setup I have to download the files.

There's audio and video embedded in them and the original files are not broken. For some reason they seem to break in the process of downloading. I'm able to recreate this in a WordPress and Drupal setting, both using modules that handle the downloads and restrict access. In Drupal I'm using Private Files V6.x-1.0. The site is on an old version of Drupal but I'm not in the position to upgrade or move to WordPress at the moment.

After a user downloads these files and tries to open them an error message pops up from PowerPoint:
"PowerPoint found a problem with the content in the file (filename). Powerpoint can attempt to repair the file. If you trust the source of this file, click Repair".

Doing so prompts another message:
"PowerPoint removed unreadable content in the file (filename [Repaired]).
You should review this presentation to determine whether any content was unexpectedly changed or removed."

After clicking OK the file opens up with several slides missing.

I can't tell if it's a matter of permissions (files are set to 604) or what the deal is. If it is I don't understand why because smaller files (~50MB - 90MB) work fine after download.

I've tried putting them in .zip files, .tar files, changing the extension from .pptx to .ppt but to no avail.

Is it possible that they break when they reach the server, or is it during the download process? If so how do I go about fixing or compensating for the issue?