The answer to your question would depend on the kind of website you have. If you have a Website Builder account, you could upload the Powerpoint files and link to them, but you wouldn't be able to require a login to view/download them. That would require the use of a database, which isn't available within Website Builder. For more information on uploading and linking to files, please see this article.
If you've built the site using another method, you may be able to do exactly what you're trying to do. Others in the community might have additional suggestions if you can give insight into how your website is built.
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