I didn't manage to test this but Receipt.pdf is been hard-coded in CiviCRM Core and if you need to change the name of the attachment then you need to look into the below file and you need to override the file to achieve your change
.../wp-content/plugins/civicrm/civicrm/CRM/Contribute/Form/Contribution/confirm.php
'PDFFilename' => 'receipt.pdf' - Line 1274
Also look into the link before overriding the file
Can I override core classes or PHP files with custom versions?Can I override core classes or PHP files with custom versions?
I guess this helps!
