As usual @mikeyevin is spot on.
To add a bit more color to this…
Webflow isn’t designed to be a download file host. One of the things they are designed to be is a website host that creates snappy sites.
One of the ways they do this is by taking all the files you upload into the asset manager, as well as select fields within the CMS, and re-upload them to their global CDN - under their own domain and a hacked up filename.
This isn’t bad.
It’s part of the magic that delivers content fast by hosting those files in regions physically closest to your website visitor. Fast downloads.
This become a problem, as you’ve discovered, under your use-case.
There are some solutions depending upon your needs and skill level.
Easiest but may not solve the problem.
You can use the native Webflow file uploader, but it has the same problem AFAIK. If you host it under your own domain that may not be the case, worth exploring.
Easier but has limitations.
You can use sites like Microsoft OneDrive or Google Drive to host. These are easy solutions but they all have their own quarks and limitations that aren’t obvious until you need more than they offer.
You can use solutions made for this like Cloudinary - Pricing and Plans and https://uploadcare.com/pricing. They both have free & paid tiers. Both allow custom domains if you need it.
While these last two options have less limitations than the first two, limitations do still exist.
More difficult, but (essentially) no limitations.
You can use a free Amazon s3 account. This is probably the most difficult option, but it gives you the most flexibility.
Huge file size downloads, custom domains, easy way to either force the display of your file inline (within the browser) or force a download, etc… Removing limitations of those other options, with the negative side-effect that it’s more complicated to setup.
Hope that helps!