In the above code we have defined our module with the version 0.1.1 and
then in global we have added new order totals in invoice PDF in admin, in which we have given in <source_field> tag BaseFeeAmount, which is the field name of our custom fee which we want to add on pdf total.
Now, new model is created to show the new total in the pdf. Create a model with the name Totalpdf.
and add the below code in it.