The allowed file extensions is set in the HTMLEditor\editor\filemanager\connectors\php\config.php file.
By default , the pdf extension is included for file uploads. to upload a file, click on the link button, the click the upload tab.
In a lot of cases where you cannot upload a file that is on the allowed extensions list, it is because of PHP upload settings such as max_file_size or post_max_size. to see if this is the case, try uploading a very small pdf of say 10k. This will help rule out the php upload settings.