The create PDF of Quote is generated based on the fields in the object and so this change must be made at field level. And it looks like it is not possible to change the date format to DD/MMM(with letters)/YYYY at field level from out of the box functionalities. There is a salesforce Idea for the same.
However, you can try the workaround as below:
You would ...
Effective quantity used in computing totals. Same as quantity unless the product is block-priced or covered by the Slab discount schedule, or an existing subscription.
For amendment quotes, this field is equal to (Quantity) - (Prior Quantity).