Like the header and footer, most of the documents in OpenERP are built in RML. RML is a flexible markup language created by ReportLab Europe Ltd. One of the primary features of this language is it can be used to produce PDF files. Adobe's PDF format is widely accepted, and OpenERP utilizes this format as the primary method of providing documents to the OpenERP end user.
RML provides the template that OpenERP requires to produce the required document. By modifying the RML you can customize reports to your liking. For further information on RML, review the documentation at http://www.reportlab.com/docs/rml2pdf-ds.pdf.