How can I change default pdf template serttings that is used while converting a form to a pdf file? (like margins, character type, size, etc)

The PDF can be customized using the customization tool.  Here is more information regarding where to find it:

https://www.jotform.com/help/384-How-to-Customize-PDF-Submissions-Report

If you want to use a similar layout for each form, save the customized PDF as a template, then use that template for your other forms: