If it's the same process, it should be pretty straightforward to use the same file upload field on multiple forms.  The form where it is originally uploaded would be an editable (and probably required) field.  On the forms where you just wanted the existing upload to be viewable, you would make the field read-only.  Then it'll appear as a link that the user can click, which downloads it.

I believe you should also be able to attach the document to an email activity in your process, similar to how you would include a copy of the submitted form or how you would reference field variables in the email.