Is it possible for someone to go back to a form that he / she left incomplete?

If someone began filling a form but did not finish because he /she could not upload a document- can it be saved and completed later?

Yes, you can do that, and there are actually two different ways to do it:

1. How to Save Forms to Continue Later

2. How to Enable Auto Fill Feature

You can see more about each on this forum thread:

How can we setup the jotform to allow our users to continue later?

We are however here if you have any questions that you want us to help you with so just let us know about them :)