Create a Custom Transaction Column Field :
Navigate to Customization >Lists, Records, & Fields > Transaction Line Fields > New
Give the label and id to the field
Select the type as Currency.
Uncheck the Store Value Checkbox.
Click Applies To Sale Item: Enter Checkmark.
In Validation & Defaulting, Enable the formula box and give the formula as the following
ROUND({amount}/{quantity},3)
Click Save
Go to the Advanced PDF Template and add this custom field Instead of the standard Item rate field.