Error when editing bill as pdf

1/ Is the procedure to edit such invoice is editing it as PDF from the "customer invoice" vue ?

2/ When using this link i got an error : "2020-02-13 10:32:36.854 ***** ERROR ***** [V8.3.2] cause = Undefined property: Bill::$tax" and the pdf is not displayed properly

3/ Is there a native way to customize the invoice with template (Company Logo, texte befre/after the billing details, ...) or should we use the pluggin Template report ?

4/ I woud like to use the "Invoice according to produce workload" mode, on project with tens of people working on tens of activities ... The only way i found to generate a bill is to add for each people, as many line as existing activities for a dedicated period, it tooks me hours. Is there a way to automatically add non-empty billing lines (including all people and all activities) just selecting a period ?

For 2/ I'm however still unable to retrieve the tax field from Bill Object and i do not undestand why:

$bill = new Bill();
$bill->tax; returns an error "Undefined property: Bill::$tax"" while the tax column exists and value is not null in the bill table.