1. The invoice has the Employee (Full name) and Employee (Initials) standard fields available under Printing fields > Columns when customizing the form so that it may be enabled to show up on PDF form.
2. To have the said fields available and to have it set to Show = T, first, the Time Track feature needs to be enabled under Setup > Company > Enable Features > Employees tab > Time & Expenses Section
3. After enabling the said feature, automatically, the Employee (Full name) and Employee (Initials) fields will be available when customizing the invoice form.