Update answer for Piotr Cierkosz:

To do this, you need to add Landscape into your custom paperformat, and then, call it in your report xml

Full code:

    

<!-- custom pager format -->
<record id="paperformat_student_report" model="report.paperformat">
    <field name="name">Custom Paperformat for the Student report</field>
    <field name="default" eval="False"/>
    <field name="disable_shrinking" eval="True"/>
    <field name="format">A4</field>
    <field name="page_height">0</field>
    <field name="page_width">0</field>
    <!-- this one is to make the pdf to become horizontal, if landscape=False, pdf will become vertical -->
    <field name="orientation">Landscape</field>
    <field name="margin_top">0</field>
    <field name="margin_bottom">8</field>
    <field name="margin_left">0</field>
    <field name="margin_right">0</field>
    <field name="dpi">96</field>
</record>

<!-- pdf version -->
<record id="action_report_student" model="ir.actions.report" style="color:#e8bf6a;">>
    <!-- this field will show as name of the button,
    also will be the name of exported file, unless we use print_report_name as below -->
    <field name="name">Students</field>
    <field name="model">school.students</field>
    <field name="report_type">qweb-pdf</field>
    <!-- In single report, this print_report_name will be use as dynamic name of the file.
    But in multiple reports, this will not work, it will use <field name="name"> as file name.
    So we should use print_report_name for single report, and <field name="name"> to multiple reports.
    -->
    <field name="print_report_name">'Student - [%s]' % (object.sequence).replace('/', '')</field>
    <!-- the view file is declare in this variable -->
    <field name="report_name">school.report_student</field>
    <!-- not sure what this for -->
    <field name="report_file">school.report_student</field>
    <!-- not sure what this for -->
    <field name="binding_model_id" ref="model_school_students"/>
    <field name="binding_type">report</field>
    <!-- custom pager format -->
    <field name="paperformat_id" ref="paperformat_student_report"/>
</record>

 
    

If you call report from a python code, then try this instead:

Add .with_context(landscape=True) to your py code:

Example:

report_action = self.env.ref('school.action_report_calendar').with_context(landscape=True).report_action(self, data=data)

Hope it helped.

You can do it by changing the QWeb attribute called "Orientation":

<field name="orientation">Landscape</field>

Official documentation is here:

https://www.odoo.com/documentation/10.0/reference/reports.html#paper-format