Hi,
You guys can also try and adding a new acceptable file upload type by adding this into functions.php file;
function my_pdf($mime_types){
$mime_types['pdf'] = 'text/pdf';
return $mime_types;
}
add_filter('upload_mimes', ' my_pdf', 1, 1);
This might work and fix the above issue. The HTTP error from my experience usually was a bad plugin or permission issue with directories. Further down it could be possible that the HTTP (apache) server isn’t allow to upload certain file or doesn’t have permissions. It’s hard to know exactly without looking at the server logs to see what’s happening.
That said, just brain storming here and your HTTP error could also be a result of upload cap; check your php.ini file if you can to see what the upload_size is. You can find more information here that will help with that part to increase; https://www.wpbeginner.com/wp-tutorials/how-to-increase-the-maximum-file-upload-size-in-wordpress/
Hope any of this helps.
Thank you,
Artur