Where is the document content available

I have a form where user can upload document, I can see the documentId and the document name in localvariable in the form, but when I try to resolve the document content, like using excelreadcellsbynumber.

But it feels like the document content is not available, does it mean the document is actually not uploaded at the moment.

Since this form is a startform of process A, I try to use startProcess in this form to initiate another process B, but it seems the document is not available in process B as well.

Can anyone explain if there is a rule that document content will only be available after submit this form