Hello asma.
Please check this method.(odoo 13)
account_reports/models/account_partner_ledger.py

@api.model
def _get_lines(self, options, line_id=None):

which return List of dictionary in that Each partner detail (credit/debit ) data is there.

base on that you can calculated you balance in PDF report.
In QWEB call this method.(def _get_lines(self, options, line_id=None)

Thanks & Regards,

Email: odoo@aktivsoftware.com

Skype: kalpeshmaheshwari