You can use sprintf command to fix the problem in invoice if you want.
example : {sprintf("%.3f", $var)}
3f = 3 decimals - 2f = 2 decimals - 0f = 0 decimals etc.
$var is your variable that you want to fix.
Invoice variables are in folder pdf under prestashop root folder and they begin with invoice.
If someone wants help how to create specific invoice , don't hesitate to send me your needs. I'll be happy to help you.