Hi,

1. Create report action:
<report
    id="report_stock_picking"
    string="Stock Picking Report"
    model="stock.picking"
    report_type="qweb-pdf"
name="your_module.report_stock_picking"
file="your_module.report_stock_picking"
    attachment_use="False"
/>

2. Define Report Template:

<?xml version="1.0"?>
<template id="report_stock_picking">
    <!-- Dynamically select template based on destination -->
<t t-if="o.partner_id.is_customer">
<t t-call="your_module.report_stock_picking_landscape"/>
    </t>
    <t t-else="">
<t t-call="your_module.report_stock_picking_portrait"/>
    </t>
</template>

<template id="report_stock_picking_landscape">
    <!-- Define landscape template -->
<t t-call="report.external_layout">
        <t t-foreach="docs" t-as="o">
            <!-- Landscape content goes here -->
        </t>
    </t>
</template>

<template id="report_stock_picking_portrait">
    <!-- Define portrait template -->
<t t-call="report.external_layout">
        <t t-foreach="docs" t-as="o">
            <!-- Portrait content goes here -->
        </t>
    </t>
</template>

Hope it helps