It's possible to custom the pdf invoice.
Yes, the invoice templates can be completely customized. The templates are in the folder /usr/local/ispconfig/interface/web/billing/templates/pdf_invoice/. Ech template is a separate folder.
Make a copy of the existing template that is the nearest match to your new invoice template by creating y copy of the folder. e.g.:
cp -prf default mytemplate
The new template folder name shall only contain chars a-z, numbers 0-9 and underscores. Dont use whitepscaes or special chars or accents for the folder name.
Then edit the template files in your new folder. The new invoice template is listed automatically in the company settings.