You can use it like it in the invoice. The attachment attribute can be used to change the name of the file.

<report
        auto="False"
        id="account_invoices"
        model="account.invoice"
        name="account.invoice"
        rml="account/report/account_print_invoice.rml"
        string="Invoices"
        attachment="(object.state in ('open','paid')) and ('INV'+(object.number or '').replace('/','')+'.pdf')"
        attachment_use="True"
        usage="default"
        />

Here this works for me.

In settings/customization/low level objects/actions/reports, I searched Quotation / Order report, and edit it with following

In Attachments,

1) In attachment field ('Save As Attachment Prefix') put following code in textbox

('SO'+(object.name or '').replace('/',''))

• here 'SO' is string whatever you want to print

2) attachment_use field ('Reload from Attachment') check mark True.

you will see that attachment named like 'SO0001' under the Attachments icon, click there to see the attachments. if you want to save this file to your computer just click the button “Save as” and then a window will appear, giving you the chance to keep or change the name of the .pdf file.

I have the same problem with Odoo 8 now. I want the number of the sales order as PDF file name. Is there any solution? 

Hi,

For v8, there is a solution, might be useful for somebody. Try this module:

https://www.odoo.com/apps/modules/8.0/report_custom_filename/

For sale order, goto Settings >> Technical >> Actions >> Reports and open the sale order report.

under Download filename, you can give ${o.name}.pdf