Some files load, some return a 404 error

We’ve reviewed the permissions, and the new files appear to align with the old working files. We think it might be the .htaccess or Ngnix since we are Using Apache.

The directory is not managed by WordPress, but you have created it yourself and link to it manually. The only interface to WordPress when calling up the URLs could actually be the .htaccess file. This results in 4 ideas:

• Have you really checked whether the files in the directory exist as they are named? Pay particular attention to capitalization. Just one difference and the file supposedly does not exist.
• If an .htaccess file exists, delete it as a test or rename it. Are the files accessible afterwards? If so, there must be something in the file that has interfered. WordPress itself does not prevent access to external directories and files in this way.
• You actually seem to be using an nginx web server, perhaps only as a proxy for Apache. There may also be rules there that deny access.
• The file names that do not work contain interesting terms. Is there possibly a security tool in the hosting that denies access based on terms? I think it would be worth checking.

@threadi thanks for the helpful input! We determined that the issue was caused by the .htaccess file. During our recent website server migration to an Nginx server running Apache, we overlooked the .htaccess file, which contained outdated plugin code. To resolve this, we deleted the file and replaced it with the default WordPress version. This solution worked, and the PDF files are now loading correctly.