Hi,

Create a Many2many Field: The task model and the field to link documents.

class CustomTask(models.Model):

    _name = 'project.task'

    document_ids = fields.Many2many('ir.attachment', string='Documents')

<record id="view_custom_task_form" model="ir.ui.view">
<field name="name">project.task.form.view.inherits</field>
<field name="model">project.task</field>
<field name="inherit_id" ref="project.project_sharing_project_task_view_form"/>
    <field name="arch" type="xml">
<xpath expr="//notebook/page[@name='description_page']" position="after">
             <group>
                             <field name="document_ids" widget="many2many_tags"/>
             </group>
        </xpath>
    </field>
</record>

Hope it helps