Modifications to customer invoice

How can I make modifications to the customer invoice in 3.8/3.9? Such as changing the font?

Thanks.

You may need to modify in PrintOrdersToPdf of PdfService at Nop.Services > Common > PdfService.

//fonts
var titleFont = GetFont();
titleFont.SetStyle(Font.BOLD);
titleFont.Color = BaseColor.BLACK;
var font = GetFont();
var attributesFont = GetFont();
attributesFont.SetStyle(Font.ITALIC);