After pdf upload, is it possible to edit field mappings?

For example: 

• lengthen the "vendor name" field to that didn't include all characters (so it could not map to existing vendor)
• combine 2 or more fields that are the invoice number (letter + prefix + number)
• add new map  box to an odoo field.

it seems I can only select a field in odoo and then clic on any of the recorgnized fields of the pdf.

also, is there any additional documentation of this feature to "teach" the ocr and mapping of fields?

thanks,

Martin Haramburu