Hi,
You can see the report_saleorder_document in this path --> odoo-10.0/addons/sale/report/sale_report_templates.xml.
To solve the issue with the overlapping of header and page , what you can do is that, after activating developer mode, go to Settings -> Technical -> Reports -> Reports , search your report, then open it, and check which is the paper format of the report.
Once you identified the paper format of the report go to, Settings -> Technical -> Reports -> Paper format
Open the paper format and increase the Header spacing and check the report.
May this can help you out.
Thanks
Are you trying to edit the "Quotation / Order" PDF ?