how do i add an existing pdf to my form

i have an independent contractor aggreement that i want to add to my form

If you simply want to display the PDF in your form, here is how to do so:

https://www.jotform.com/help/153-How-to-Display-a-PDF-Document-on-your-Form

If there is something else you are trying to do, let usk now and we will be happy to help.

how would i make the pdf fillable like the form
...

It is not currently possible to take a PDF created outside of JotForm and convert it to a fillable PDF or convert it to a web form.  PDF's created outside of JotForm would need to be recreated manually.