If your theme is not supported by default check here if there is a compatibility mod here.
If not try pdf_invoice_common_model_1.xml
If doesn't make it, request me to make one at sirius_box-dev@yahoo.fr

Install (VQMOD) :
- delete the file /vqmod/xml/pdf_invoice_default_theme.xml
- copy the file corresponding to your theme in /vqmod/xml/ folder
- done :]

Install (OCMOD) :
- delete the file /system/pdf_invoice_default_theme.xml
- copy the file corresponding to your theme in /system/ folder
- rename this file to [filename].ocmod.xml
- go to extension > modifications, and click refresh modifications
- done :]