You can increase the upload limit for just one site without changing it globally.
In Forge, go to your site → Files → Edit Nginx Configuration and make two changes:

1. Find client_max_body_size and update it (or add it inside the server block):

client_max_body_size 8m;

2. Inside the location ~ \.php$ block, add:

fastcgi_param PHP_VALUE "upload_max_filesize=8M";
fastcgi_param PHP_ADMIN_VALUE "post_max_size=9M";

# (post_max_size should be slightly larger to account for other form data in the request.)

As for restricting it to only PDF uploads, that's not something that can be handled at the server level, since the size limit applies to the entire request before the server knows what type of file is being uploaded. The best approach is to set the 8M limit on the site as above, and then enforce the file type restriction in your application's validation. For example, in Laravel:

'file' => ['required', 'file', 'mimes:pdf', 'max:8192'],

That way, the higher limit is available only on that site, and your app ensures only PDFs are actually accepted.