Hello, the best way of modifying the report it is to develop a module and to inherit the template of base. You can find the module report in addons and in view you will find the layout, it is in this file that is the template footer of the report.
look this code:
<template id="external_layout_header" inherit_id="report.external_layout_header">
<xpath expr="//div" position="replace">
<div class="footer">
<div class="text-center" style="border-top: 1px solid black;">
<ul t-if="not company.custom_footer" class="list-inline">
<li t-if="company.phone">Phone: <span t-field="company.phone"/></li>
<li t-if="company.fax and company.phone">&bull;</li>
<li t-if="company.fax">Fax: <span t-field="company.fax"/></li>
<li t-if="company.email">&bull;</li>
<li t-if="company.email">Email: <span t-field="company.email"/></li>
<li t-if="company.website">&bull;</li>
<li t-if="company.website">Website: <span t-field="company.website"/></li>
</ul>
<t t-if="company.custom_footer">
<span t-raw="company.rml_footer"/>
</t>
<ul class="list-inline">
<li>Page:</li>
<li><span class="page"/></li>
<li>/</li>
<li><span class="topage"/></li>
</ul>
</div>
</div>
</template>