failed with status 400 and response 0
When you make a request to admin-ajax.php you need to pass an action parameter, which here is upload_pdf. If nothing on the PHP side handles this then admin-ajax.php will return a 0 and a HTTP status of 400.
Essentially, WordPress is saying it has no idea what you’re talking about.
Looking at the comments it seems you’ve copied this javascript across sites expecting it to work on its own. These other sites have an admin AJAX handler in their PHP that is missing from this site. This could be because it wasn’t copied over, a new theme, a missing plugin, etc.
You will need to search an older working site and discover the location of the admin ajax handler. This is not something that people online can help you with as it requires searching your servers files. Once you have done this you will have your answer for how to fix it, e.g. you found it in a different theme, or in a plugin you forgot to install/activate.