Hello Anton, 

In your expense report these fields are available or not if these fields available in case you can add into PDF.

If this fields not available in that case kindly add these fields into backend (database) and based on that you can add into PDF. 

approve_id = fields.Many2one('res.users', string='Approved By')
approve_date_time = fields.Datetime(string='Approval Time')

def _get_approval(self):
	​for res in self:
	​	​res.approve_id = self.env.uid
	​	​res.approve_date_time = datetime.now()

Thanks. 

You can archive this by tracking, 

Example :  We have a Approve Button, by tracking that button using following fields,

button_click_time = fields.Datetime(string="Button Click Time", readonly=True, tracking=True)
button_clicked_by = fields.Many2one('res.users', string="Button Clicked By", readonly=True, tracking=True)

And add this function for the approve button,

def action_approve_button(self):
    self.button_click_time = datetime.now()
    self.button_clicked_by = self.env.user.id 

By this we can easily show the data in a report.

Check and Clarify your doubts.