1.You can add a serial number to a PDF form.
If the form is used offline (Opened directly within Adobe Reader), you can generate an auto number from a web service.
If the form is used within Workspace ES2, you can make use of the Prepare Data Action Profile to populate this number. (Web service way also may apply)
2. You cannot add reminder to PDF. Instead, you can add reminders to Tasks assigned to Users/Groups using AssignTask Activity in Workflows.