You need to have an active domain setup on your server. If you don't have it setup yet, click where the arrows are to create the domain. Also, make sure you have "User" selected at top as the Admin panel does not have a File Manager icon by default any more.
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In your panel, click "File Manager"
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When it opens, navigate to the public_html folder of the domain / subdomain you want to serve from and drag and drop file into that folder.
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Anything above public_html in the heirarchy won't get served and won't be accessible via a browser
If you wanted somewhere like yourdomain.com/files/your_pdf.pdf then create the "files" folder in the "public_html" directory, navigate to that folder and upload your pdf files