Generally the best way to do this, is to upload the file to your server, and then store the path to it in the database.
File Upload: http://php.net/manual/en/features.file-upload.php
You need to choose a database, MySQL is a common and free option:
As mentioned in comment below, (I haven’t used it before but had a read and looks like a better option) https://mariadb.com/
PDO (recommended library to use with several types of databases): http://php.net/manual/en/book.pdo.php
Note that there are many security considerations when letting people upload files to your server: https://security.stackexchange.com/questions/32852/risks-of-a-php-image-upload-form/32853#32853
All in all, it may be longer than you think before your friend is uploading that file – but it will be worth it