Here is a Quick and Dirty way of resolving the immediate problem.
From Settings -> General -> Business Documents -> Layout, we can see that the invoice / quote PDF is using external_layout_standard QView and Odoo admin can access the XML architecture of this report. However, editing this view as an Odoo admin does not allow access to CSS files where we would be able to modify the layout for good. We can, however, hack this a bit.
Follow the steps:
Settings with Developer mode on -> Edit document layout under Business documents
Look for section
<div class="col-6" name="company_address">
<div t-field="company.partner_id" t-options="{"widget": "contact", "fields": ["address", "name"], "no_marker": true}"/>
</div>
Add Style attribute (style="font-size:14px") for inner <div>:
<div class="col-6" name="company_address">
<div t-field="company.partner_id" style="font-size:14px" t-options="{"widget": "contact", "fields": ["address", "name"], "no_marker": true}"/>
</div>
- This changes the sender company address font size. We do still need to change the recipient address font size.
- Navigate to
Settings -> Technical -> Views (under User interface) - Search for
address_layout - Add same style attbute (
style="font-size:14px") for section:
<div style="font-size:14px" name="address" t-att-class="colclass">
<t t-raw="address"/>
</div>
The end result is not the prettiest layout in the world, but it works.
The right way is apparently to inherit 'account.report_invoice_document' and then change the template code. So this is not possible with only Odoo admin UI. It requires writing a small extension.
Stack Overflow has a thread covering the same topic: https://stackoverflow.com/questions/61398717/odoo-how-to-edit-invoice-quote-layout-css
Have updated Ossi's suggested link with solution
Modify this file for purchase orders Technical -> Views -> report_purchaseorder_document Locate the section
Request for Quotation #
<h4 t-if="o.state in ['sent', 'to approve']">Purchase Order #<span t-field="o.name"/></h4>
<h4 t-if="o.state in ['purchase', 'done']">Purchase Order #<span t-field="o.name"/></h4>
<h4 t-if="o.state == 'cancel'">Cancelled Purchase Order #<span t-field="o.name"/></h4>
For RFQ modify views -> report_purchasequotation_document Request for Quotation
Note these header tags were h2's