This is currently logged as an enhancement request 105120: Transactions form PDF Layout > Item Label > Add ability to print item name / number.
As an alternative, user may create a custom transaction field:
1. Navigate to Customization > Lists, Records & Fields > Transaction Line Field > New.
2. Set Label = Item Name.
3. Set Type = Free Form Text.
4. Do not mark the Store Value checkbox.
5. Set Description = To show Item Name on the form.
6. On the Applies To tab > mark Sale Item, and Print on Packing Slip.
7. On the Sourcing & Filtering tab> set Source List = Item and Source From = Name.
8. Click Save and Apply to Forms.