What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
If the Invoice is already a fillable PDF, then you can import it to us and use it to collect data as a web form.
But, if the form is just a PDF, then what you can do is re-create it manually using our form builder. After that you can convert it to a fillable PDF, here's a guide on how to convert it to fillable PDF: http://www.jotform.com/help/312-Fillable-PDF-Everything-you-need-to-know-about-JotForm-Fillable-PDF-Creator.
Do let us know if you need more information on this.