Hello

In odoo v10, it seems to be super easy... That's what we did with google font Roboto :

- Inherit css with google :

<xpath expr="." position="inside">
    <link href="https://fonts.googleapis.com/css?family=Roboto|Roboto+Condensed" rel="stylesheet"/>
</xpath>

- Inherit the reports with font-family :

<xpath expr="//div[@class='page']" position="attributes">
    <attribute name="style">font-family: 'Roboto', sans-serif;</attribute>
</xpath>

- Inherit the header and footer with font-family :

<xpath expr="//div[@class='footer']" position="replace">
    <div class="footer" style="font-family: 'Roboto Condensed', sans-serif;">
...

It works with wkhtmltopdf 0.12.1

In case someone is looking for an Odoo v12 approach:

If you just need your custom font to be rendered in your HTML reports. In your custom css file you can:

body{    
    font-family: 'Montserrat', sans-serif;
}

And then, in your assets.xml file you could inherit the report_assets_common in this way:

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
        <template id="report_assets_common" name="Technical Service Assets Reports" inherit_id="web.report_assets_common">		
            <xpath expr="//link[last()]" position="after">			
                <link href="https://fonts.googleapis.com/css?family=Montserrat" rel="stylesheet"/> /*Here your chosen Google Font or any other*/		
                <link rel="stylesheet" href="your_custom_module/static/src/css/report_styles.css"/> /*This is your custom css file*/
            </xpath>	
        </template>
</odoo>

Finally add both files if not already there in your __manifest__.py:

'data': [
    'views/assets.xml',
],
'css': [
    'static/src/css/report_styles.css',
],

But this doesn't work that easy with pdf rendering. In that other case, following the @Cyril concept:

 This is for PDF printing reports:

Same as before, append your new fonts to the assets:

<?xml version="1.0" encoding="utf-8" ?>
<odoo>        
    <template id="report_assets_common" name="Technical Service Assets Reports" inherit_id="web.report_assets_common">		           
        <xpath expr="//link[last()]" position="after">			                
            <link href="https://fonts.googleapis.com/css?family=Montserrat" rel="stylesheet"/> /*Here your chosen Google Font or any other*/		                
            <link rel="stylesheet" href="your_custom_module/static/src/css/report_styles.css"/> /*This is your custom css file*/            
        </xpath>	        
    </template>
</odoo>

You could inherit your chosen layout, in this case I'll take the external_layout_standard, and assign the styles to those part of the template you want to be affected. I'll change everything, so I'll select the header, article and the footer. Don't forget to appent to the __manifest__.py and upgrade your module after all.

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <data>
        <template id="external_layout_standard" inherit_id="web.external_layout_standard">            
            <xpath expr="//div[@class='header']" position="attributes">
                <attribute name="style">font-family: 'Montserrat', sans-serif;</attribute>
            </xpath>
            <xpath expr="//div[@class='article']" position="attributes">
                <attribute name="style">font-family: 'Montserrat', sans-serif;</attribute>
            </xpath> 
            <xpath expr="//div[@class='footer']" position="attributes">
                <attribute name="style">font-family: 'Montserrat', sans-serif;</attribute>
            </xpath>
        </template>
    </data>
</odoo>

Here is the complete xml for inheriting css :

<template id="my_module.report_assets_common" name="my_module report assets common" inherit_id="report.assets_common">
    <xpath expr="." position="inside">
        <link href="https://fonts.googleapis.com/css?family=Roboto|Roboto+Condensed" rel="stylesheet"/>
    </xpath>
</template>

Generically, you can use a custom font using  @font-face  in your CSS. Here’s a very basic example:
@font-face {
font-family: 'YourFontName'; /*a name to be used later*/
src: url('http://domain.com/fonts/font.ttf'); /*URL to font*/
}

Then, trivially, to use the font on a specific element:

`.classname {`
`    font-family: 'YourFontName';`
`}`

.classname {
    font-family: 'YourFontName';
}
``
(.classname is your selector).

Note that certain font-formats don’t work on all browsers; you can use some generator to avoid too much effort converting.

You can find a nice set of free web-fonts provided by Google Fonts (I can’t insert more than 1 link so you can go to Google fonts) (also has auto-generated CSS  @font-face  rules, so you don’t have to write your own).

while also preventing people from having free access to download the font, if possible

Nope, it isn’t possible to style your text with a custom font embedded via CSS, while preventing people from downloading it. You need to use images, Flash, or the HTML5 Canvas, all of which aren’t very practical.

I hope that helped!

You can try check this module. https://apps.odoo.com/apps/modules/15.0/advanced_report_templates/