You can define any name you want for the PDF documents you create using LSP.expert.
You can do that at two different places: in the sending screen itself (just before sending your document) and at the client level (Clients tab under People).
In the sending screen:
When you're in an invoice, for example, you first have to click on Actions > Email invoice (or Email quote, in the case of a quote).
Once you're in the sending screen, you simply have to click on the little pencil next to the name of the document to be sent (at the bottom). Please note that the changes you make here in the name format will be automatically saved at the client level (meaning that all your future documents for this client will be created using the same name format).
At the client level:
To adapt the file name format at the client level, you have to edit the corresponding field for the client concerned. All your documents for this client will then be created using this name format.