Hi Ben,
The filename is generated around line 122 of modules/AOS_PDF_Templates/generatePdf.php (https://github.com/salesagility/SuiteCRM/blob/hotfix-7.2.3/modules/AOS_PDF_Templates/generatePdf.php#L122).
You can change the static string at the start by changing the LBL_PDF_NAME label in the appropriate module (i.e. in invoices).
It’s possible to change the naming but would require some customisation. Making the changes in the above file isn’t upgrade safe so it’s better to create a copy of this file and place it into ‘custom/modules/AOS_PDF_Templates/generatePdf.php’ and make the changes you need there. This file isn’t automatically picked up unfortunately. This means you’ll also need to edit the $entry_point_registry to point to the new version of generatePdf (have a look in ‘custom/Extension/application/Ext/EntryPointRegistry/*’ for examples of how to change this).
Hope this helps,
Jim