Can’t use images, CSS through HTTPS.

Hi!

Your plugin is great but I have one problem. I paste an url to CSS and this is what I’ve got:

Warning: file_get_contents(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed in /data/web/virtuals/215931/virtual/www/cz/wp-content/plugins/send-pdf-for-contact-form-7/views/send-pdf-admin.php on line 247

Warning: file_get_contents(): Failed to enable crypto in /data/web/virtuals/215931/virtual/www/cz/wp-content/plugins/send-pdf-for-contact-form-7/views/send-pdf-admin.php on line 247

Warning: file_get_contents(https://mywebsite.com/documents/assets.css): failed to open stream: operation failed in /data/web/virtuals/215931/virtual/www/cz/wp-content/plugins/send-pdf-for-contact-form-7/views/send-pdf-admin.php on line 247

Any solution?
Thanks