I had to make a couple of changes for the PDF invoices to show my preferred date format:
- Change the date format in the language *my profile* is using. I added the month abbreviation to make it easier to confirm I had the desired date i.e. instead of 1/1/24 I now see 1/Jan/24.
- I had to change the language assigned to my customer by editing the Contact and setting their language to match my preferred language.
Additionally, I found that once a PDF has been generated for the Invoice, Odoo will not generate a new PDF reflecting your changes to dates until you delete the PDF from the Invoice (it's an attachment accessible from below the invoice details).