Is it possible to create a personalized pdf invoice template based in existing templates?
Yes, just make a copy of one of the xisting templates. Ensure that the dircetory name has no white spaces as in your first attempt, use underscore "_" instead.
Then open the make_pdf_invoice.php file in your newly created directory and edit the calssname to match the new name. There are instructions in the file:
Here we extend tfpdf and create a new class. the name of the Class has to be
PDFInvoice_ plus the name of the template, in this case "default" so the full
name of the class is "PDFInvoice_default".
class PDFInvoice_default extends tFPDF