How to change invoice pdf's?

Hi,

No, customizing the invoice with a custom field is the wrong approach.

Since you're using the PDF invoice plugin to generate an invoice with a PDF format, you want to follow the instructions provided on the page relative to this plugin:

If you want to customize the layout you've selected in the settings of the plugin, you'll need to :
Connect to your website via FTP or your cpanel's file manager
Download the file of the layout in the folder plugins/hikashop/attachinvoice/attachinvoice/
Modify the file locally with Notepad++ or Visual Studio Code
Upload the modified file to media/com_hikashop/plugins/
That way, you'll have your modifications as an override of the layout, so you won't lose them when you update the plugin in the future.